Nonvolatile semiconductor memory device employing transistors having different gate withstand voltages for enhanced reading speed

ABSTRACT

A semiconductor device includes a plurality of nonvolatile memory cells ( 1 ). Each of the nonvolatile memory cells comprises a MOS type first transistor section ( 3 ) used for information storage, and a MOS type second transistor section ( 4 ) which selects the first transistor section. The second transistor section has a bit line electrode ( 16 ) connected to a bit line, and a control gate electrode ( 18 ) connected to a control gate control line. The first transistor section has a source line electrode ( 10 ) connected to a source line, a memory gate electrode ( 14 ) connected to a memory gate control line, and a charge storage region ( 11 ) disposed directly below the memory gate electrode. A gate withstand voltage of the second transistor section is lower than that of the first transistor section. Assuming that the thickness of a gate insulating film of the second transistor section is defined as tc and the thickness of a gate insulating film of the first transistor section is defined as tm, they have a relationship of tc&lt;tm.

TECHNICAL FIELD

The present invention relates to a semiconductor device having anonvolatile memory, and particularly to a technology for reading memoryinformation at high speed, e.g., a technology effective if applied to aflash memory or a microcomputer or the like including the flash memoryprovided on-chip.

BACKGROUND ART

As nonvolatile memory cells, may be mentioned, a split gate type memorycell and a stack gate type memory cell. The split gate type memory cellcomprises two transistors of a memory MOS type transistor thatconstitutes a memory section, and a selection MOS type transistor forselecting its memory section to thereby fetch information therefrom. Asa known document, there is known a technology described in1994-Proceedings of IEEE, VLSI, Technology Symposium, pp 71–72. Astructure and operation of a memory cell described therein will beexplained in brief. This split gate type memory cell comprises a source,a drain, a floating gate and a control gate. As the injection ofelectrical charges into the floating gate, may be mentioned a sourceside injection system using the generation of hot electrons. The chargesstored in the floating gate are ejected from a pointed end of thefloating gate to the control gate. At this time, there is a need toapply a high voltage of 12 volts to the control gate. The control gatethat functions as a charge ejection electrode, serves even as a gateelectrode of a reading selection MOS type transistor. A gate oxide filmfor a selection MOS type transistor section is a deposited oxide film,which functions even as a film for electrically isolating the floatinggate and a gate electrode of the selection MOS type transistor. As otherknown technologies related to the split gate type memory cell, there areknown, for example, U.S. Pat. Nos. 4,659,828 and 5,408,115, JapaneseUnexamined Patent Publication No. Hei 5(1993)-136422, etc.

The stack gate type memory cell comprises a source, a drain, and afloating gate and a control gate stacked on a channel forming region.The generation of hot electrons is used for the injection of electricalcharges into the floating gate. The electrical charges stored in thefloating gate are ejected toward a substrate. At this time, there is aneed to apply a negative high voltage of −10 volts to the control gate.Reading is performed by applying a read voltage like 3.3 volts to thecontrol gate. The stack gate type memory cell has been described inJapanese Unexamined Patent Publication No. Hei 11(1999)-232886, etc.

In terms of the speeding up of data processing, the speeding up of aread operation of a nonvolatile memory device becomes important even tothe nonvolatile memory device. In the split gate type memory cell, thegate electrode of the selection MOS transistor is configured so as tofunction even as an erase electrode. Therefore, a gate insulating filmhad no other choice but to set its thickness to the same thickness asthat of a write/erase-voltage control high-voltage MOS transistor inorder to ensure a withstand voltage therefor. Thus, Gm (mutualconductance defined as current supply capacity) of the selection MOStransistor becomes small, so the split gate type memory cell is hardly astructure wherein a read current can be obtained sufficiently. Ifnothing is done, then the split gate type memory cell is not fit for ahigh-speed operation under a low voltage. Since a thick gate oxide filmfor realizing a high withstand voltage is adopted for the control gateto which a high voltage is applied upon write/erase operations, itreduces Gm at a read operation, so the stack gate type cell is hardly astructure wherein a read current can be ensured sufficiently.

U.S. Pat. Nos. 4,659,828 and 5,408,115 of the known documentsrespectively describe the invention related to the write/eraseoperations but do not refer to an improvement in the performance of theread operation. Further, although Japanese Unexamined Patent PublicationNo. Hei 5(1993)-136422 of the known document discloses a shape mostanalogous to that of the present invention, it shows the inventionrelated to a method of insulating two gate electrodes adjacent to eachother, and does not disclose read performance. A nonvolatile memorydevice unprovided for the prior art is needed which is adapted to alogical operation device brought to high performance.

A structure has been adopted wherein bit lines are hierarchized intomain and sub bit lines, only a sub bit line connected with a memory cellto be operated and selected is selected and connected to itscorresponding main bit line, and the parasitic capacity of the bit lineby the memory cell is apparently reduced, whereby a high-speed readoperation is realized. However, it has been found out by the presentinventors that there is a fear that where it is necessary to apply ahigh voltage even to a bit line upon writing as in the stack gate typememory cell, a MOS transistor for selectively connecting a sub bit lineto its corresponding main bit line must be brought to high withstanding,whereby Gm of a read path is further reduced and the speeding up by ahierarchized bit line structure based on the main/sub bit lines will notfunction sufficiently.

An object of the present invention is to eliminate a thick-filmhigh-voltage MOS transistor that impairs speeding up, from a memoryinformation read path.

Another object of the present invention is to provide a semiconductordevice capable of reading memory information from a nonvolatile memorycell at high speed.

The above, other objects and novel features of the present inventionwill become apparent from the description of the present Specificationand the accompanying drawings.

DISCLOSURE OF THE INVENTION

Summaries of representative ones of the inventions disclosed in thepresent application will be explained in brief as follows:

[1] A semiconductor device includes a plurality of nonvolatile memorycells (1). Each of the nonvolatile memory cells comprises a MOS typefirst transistor section (3) used for information storage, and a MOStype second transistor section (4) which selects the first transistorsection. The second transistor section has a bit line electrode (16)connected to a bit line (BL), and a control gate electrode (18)connected to a control gate control line (CL). The first transistorsection has a source line electrode (10) connected to a source line, amemory gate electrode (14) connected to a memory gate control line (ML),and a charge storage region (11) disposed directly below the memory gateelectrode. A gate withstand voltage of the second transistor section islower than that of the first transistor section. In other words,assuming that the thickness of a gate insulating film (17) of thecontrol gate electrode of the second transistor section is defined as tcand the thickness of a gate insulating film (11, 12, 13) of the memorygate electrode of the first transistor section is defined as tm, theyhave a relationship of tc<tm. Here, MOS is a generic name for aninsulated gate field effect type transistor structure.

According to the above, when the second transistor section of thenonvolatile memory cell is brought to an on state upon a data readoperation, memory information is read out to the corresponding bit lineaccording to whether the current flows in accordance with a thresholdvoltage state of the first transistor section. The second transistorsection is thinner than the first transistor section in gate oxide-filmthickness and lower than it in gate withstand voltage too. Therefore, ascompared with a case in which both a memory holding MOS transistorsection and a selection MOS transistor section are formed at a highwithstand voltage, a relatively large Gm can be easily obtained at arelatively low gate voltage with respect to the selection MOS transistorsection, and the current supply capacity of the entire nonvolatilememory cell, i.e. Gm can be relatively increased, thereby contributingto the speeding up of a read speed.

Upon the operation of setting a relatively high threshold voltage to thefirst transistor section, for example, a high voltage is applied to itsmemory gate electrode to turn on the second transistor section, therebyallowing a current to flow from the source line to the bit line, wherebyhot electrons generated in the vicinity of the charge storage region onthe control gate side may be retained in the charge storage region. Uponthe operation of setting a relatively low threshold voltage to the firsttransistor section, for example, a high voltage is applied to its memorygate electrode to turn on the second transistor section, thereby settingthe bit line electrode and the source line electrode to a circuit'sground potential, whereby the electrons retained in the charge storageregion may be ejected toward the memory gate electrode. Thus, theoperation of setting the relatively low threshold voltage or therelatively high threshold voltage to the first transistor section can berealized without applying the high voltage to the control gate controlline and the bit line. This guarantees that the gate withstand voltageof the second transistor section may be relatively low.

It is desired that in order to make it hard for the charges stored inthe charge storage region to leak into the control gate electrode, arelationship of tm≦ti is established assuming that the thickness of aninsulating film (9) between the control gate electrode and the chargestorage region is defined as ti, for example.

In order to assure a low gate withstand voltage of the second transistorsection on a device structure basis, for example, a high-densityimpurity region (30) may be prevented from being formed between the bitline electrode and the source line electrode formed in a well region.The high-density impurity region is a diffused region of an impurity. Inthe case of a nonvolatile memory cell comprising a series circuit of amemory holding MONOS section and a selection MOS transistor section,series-connected nodes of both transistor sections are configured as adiffusion region (source-drain region) common to both. When the commondiffusion region common to both the transistor sections is interposedtherebetween, a high voltage is applied to the MONOS section at writingto form a channel, so that the high voltage on the MONOS side is appliedto the selection MOS transistor section from the channel via thediffusion region common to both the transistor sections. It is thusessential that the selection MOS transistor section is at a highwithstand voltage in the case of the MONOS type memory cell.

The charge storage region may adopt a conductive floating gate electrodecovered with an insulating film, or may adopt a charge trap insulatingfilm covered with an insulating film, a conductive fine particle layercovered with an insulating film, or the like.

A switch MOS transistor (19) is provided which is capable of connectingthe bit line to its corresponding global bit line (GL), and a dividedbit line structure (hierarchical bit line structure) may be adoptedtherefor. The divided bit line structure contributes to the fact thatupon a read operation, only some nonvolatile memory cells are connectedto the corresponding global bit line to thereby apparently reduce theparasitic capacity of the bit line and further speed up the readoperation. Since, at this time, the high voltage may not be applied tothe bit line upon erase/write operations, the gate oxide-film thicknessof the switch MOS transistor may be formed thinner than that of thefirst transistor section. In short, it is easy to give a relativelylarge current supply capacity to the switch MOS transistor. Further, itis possible to ensure the speeding up of the read operation by thedivided bit line structure.

[2] As a further detailed aspect, the semiconductor device includes afirst driver (21) which drives the control gate control line, a seconddriver (22) which drives the memory gate control line, a third driver(23) which drives the switch MOS transistor to an on state, and a fourthdriver (24) which drives the source line. The first driver and the thirddriver use a first voltage as an operating power supply, and the seconddriver and the fourth driver use a voltage higher than the first voltageas an operating power supply.

The semiconductor device has a control circuit (76) which when thethreshold voltage of the first transistor section is taken high, setsthe operating power supply of the first driver to a first voltage, setsthe operating power supply of the fourth driver to a second voltagehigher than the first voltage, and sets the operating power supply ofthe second driver to a third voltage higher than the second voltage,thereby enabling injection of hot electrons into the correspondingcharge storage region from the bit line electrode side.

When the threshold voltage of the first transistor section is taken low,the control circuit sets the operating power supply of the second driverto a fourth voltage higher than the third voltage, thereby ejectingelectrons from the charge storage region to the corresponding memorygate electrode.

The first transistor section whose threshold voltage has been lowered,may be set to, for example, a depletion type. The first transistorsection whose threshold voltage has been rendered high, may be set to,for example, an enhancement type. The memory gate electrode at the readoperation may be set to a circuit's ground voltage. Since the secondtransistor section that selects the first transistor section is providedwith respect to the first transistor section, a selection free of averify operation strict on writing and erasure is also enabled.

When information stored in the nonvolatile memory cell is read, thecontrol circuit may set the operating power supply of the first driverto a first voltage and set the memory gate electrode and the source lineelectrode to a circuit's ground potential. The direction of current atthe read operation results in the direction thereof from the bit line tothe source line.

When the information stored in the nonvolatile memory cell is read, thecontrol circuit may set the operating power supply of the first driverto a first voltage and set the memory gate electrode and the bit lineelectrode to a circuit's ground potential. The direction of current atthe read operation results in the direction thereof from the source lineto the bit line contrary to the above.

The semiconductor device described above may be not only the discretenonvolatile memory but also a semiconductor device such as amicrocomputer with the nonvolatile memory provided on-chip, a dataprocessor. For example, the semiconductor device further has a logicoperation unit (61) which performs a logical operation with the firstvoltage as an operating power supply.

When viewed from a layout standpoint, each of the first driver and thethird driver may receive an address decode signal (51) so that anoperation thereof is selected, and each of the second driver and thefourth driver may receive the output (52) of the first driver so that anoperation thereof is selected.

The first driver and the third driver may be disposed on one side andthe second driver and the fourth driver may be disposed on the otherside, with at least one the nonvolatile memory cell array (50) beinginterposed therebetween. It is possible to separate drivers eachoperated with a high voltage as an operating power supply and circuitseach operated with a relatively low voltage as an operating power supplyfrom one another.

In the memory array, memory gate control lines (ML) are formedintegrally with memory gate electrodes, and low resistance metal layers(MGmt) may be configured so as to be laminated over polysilicon layers(MGps), respectively. Control gate control lines (CL) may also beconfigured integrally with their corresponding control gate electrodes.Further, low resistance metal layers (CGmt) may be configured so as tobe laminated over their corresponding polysilicon layers (CGps). Thus,the resistance of wiring can be reduced.

Discharge MOS transistors 53 for respectively causing the memory gatecontrol lines to be conducted to the circuit's ground potential inresponse to a read operation may be provided at different positions ofthe memory gate control lines. It is possible to make rapid transitionto a read operation enable state.

As the switch MOS transistor placed under the divided bit linestructure, may be adopted a p channel type MOS transistor (19 p). It isthus possible to prevent a signal level from being reduced by thethreshold voltage of the switch MOS transistor and satisfactorily copewith a voltage reduction in read signal level to the corresponding bitline. However, even if an attempt to set the bit line to the circuit'ground potential is made when the threshold voltage of the correspondingnonvolatile memory cell is made high, the potential of the bit line doesnot reach a level lower than the threshold voltage of the p channel typeswitch MOS transistor. In order to solve it, the switch MOS transistormay be made up of CMOS transfer gates (19 p, 19 n).

n channel type discharge MOS transistors (20 n) each switch-operatedcomplementarily to the switch MOS transistor may be provided at theircorresponding bit lines. Thus, when the corresponding bit line isselected via the switch MOS transistor, the bit line is perfectlydischarged by its corresponding discharge MOS transistor, so that thelevel of a global bit line precharged prior to the start of reading canbe prevented from undesirably varying, thereby contributing tostabilization of a sense operation of a read signal and speeding up of aread operation.

[3] The present invention will be grasped from the viewpoint slightlydifferent from the above. A semiconductor device has nonvolatile memorycells (1) arranged in a semiconductor substrate (2) in matrix form. Eachof the nonvolatile memory cells includes in the semiconductor substratea source line electrode (10) connected to a source line (SL), a bit lineelectrode (16) connected to a bit line (BL), and channel regionsinterposed between the source line electrode and the bit line electrode.Further, the nonvolatile memory cell includes over the channel regions acontrol gate electrode (18) disposed near the bit line electrode via afirst insulating film (17) and connected to a control gate control line(CL), and a memory gate electrode (14) disposed via a second insulatingfilm (12, 13) and a charge storage region (11), electrically separatedfrom the control gate electrode (18) and connected to a memory gatecontrol line (ML). The withstand voltage of the first insulating film islower than that of the second insulating film.

It becomes easy to obtain a relatively large Gm at a relatively low gatevoltage with respect to a selection MOS transistor having the controlgate electrode. Current supply capacity of the whole nonvolatile memorycell, i.e., Gm can be relatively made large, thereby contributing to thespeeding up of a read speed.

In order to relatively set high a threshold voltage of the nonvolatilememory cell as viewed from the memory gate thereof, for example, a highvoltage is applied to its memory gate electrode to turn on the controlgate electrode side, thereby allowing a current to flow from the sourceline to the bit line, whereby electrons generated in the vicinity of thecharge storage region on the control gate electrode side may be retainedin the charge storage region. In order to set a relative low thresholdvoltage in reverse, for example, a high voltage is applied to its memorygate electrode to turn on the control gate electrode side, therebysetting the bit line electrode and the source line electrode to acircuit's ground potential, whereby the electrons retained in the chargestorage region may be ejected toward the memory gate electrode. Thus,the operation of setting the relatively low threshold voltage or therelatively high threshold voltage to the nonvolatile memory cell can berealized without applying the high voltage to the control gate controlline and the bit line. This guarantees that the gate withstand voltageon the control gate electrode side may be relatively low.

A semiconductor device according to a further specific aspect, havingthe nonvolatile memory cells includes control gate drivers each of whichdrives the control gate control line, memory gate drivers each of whichdrives the memory gate control line, and source drivers each of whichdrives the source line. At this time, the control gate driver may use afirst voltage as an operating power supply, and each of the memory gatedriver and the source driver may use a voltage higher than the firstvoltage as an operating power supply.

The semiconductor device has a control circuit which sets an operatingpower supply of the control gate driver to a first voltage, sets anoperating power supply of the source driver to a second voltage higherthan the first voltage, and sets an operating power supply of the memorygate driver to a third voltage greater than or equal to the secondvoltage when a threshold voltage of the nonvolatile memory cell asviewed from the memory gate electrode is rendered high, thereby enablinginjection of electrons into the corresponding charge storage region fromthe bit line electrode side.

The control circuit sets the operating power supply of the memory gatedriver to a fourth voltage greater than or equal to the third voltagewhen the threshold voltage of the nonvolatile memory cell as viewed fromthe memory gate electrode is rendered low, thereby ejecting electronsfrom the charge storage region to the corresponding memory gateelectrode.

When information stored in the nonvolatile memory cell is read, thecontrol circuit sets the operating power supply of the control gatedriver to a first voltage and sets the memory gate electrode and thesource line electrode to a circuit's ground potential. The direction ofcurrent at this read operation results in the direction thereofextending from the bit line to the source line. Incidentally, at thistime, the memory gate electrode may be a voltage higher than the groundpotential.

When information stored in the nonvolatile memory cell is read, thecontrol circuit sets the operating power supply of the control gatedriver to a first voltage and sets the memory gate electrode and the bitline electrode to a circuit's ground potential. The direction of currentat this read operation results in the direction thereof from the sourceline to the bit line contrary to the above. In a manner similar to theabove, the memory gate electrode at this time may be a voltage higherthan the ground potential.

The semiconductor device may be not only the discrete nonvolatile memorybut also a microcomputer with the nonvolatile memory provided on-chip, adata processor or the like. For example, the semiconductor device has alogic operation unit which performs a logical operation with the firstvoltage as an operating power supply.

The control gate driver may be one inputted with an address decodesignal so that an operation thereof is selected, and each of the memorygate driver and the source driver may be one based on the output of thecontrol gate driver so that an operation thereof is selected.

The control gate drivers may be disposed on one side and the memory gatedrivers and the source drivers may be disposed on the other side, withat least one array of the nonvolatile memory cells being interposedtherebetween. It becomes easy to separate drivers each operated with ahigh voltage as an operating power supply and circuits each operatedwith a relatively low voltage as an operating power supply from oneanother.

In the array of the nonvolatile memory cells, memory gate control linesmay be formed integrally with memory gate electrodes, and low resistancemetal layers may be formed with being laminated over polysilicon layersrespectively. Thus, the resistance of wiring can be reduced.

Attention is paid to a reduction in chip occupied area formed by thememory gate driver and the source driver. In the array of thenonvolatile memory cells, the memory gate drivers (22A) may preferablybe shared in plural units of the memory gate control lines paired withthe control gate control lines, and the source drivers (24) maypreferably be shared in plural units of the source lines paired with thecontrol gate control lines. At this time, the number of the memory gatecontrol lines shared by the corresponding memory gate driver maypreferably be less than or equal to the number of the source linesshared by the corresponding source driver. For instance, when thecurrent is caused to flow between the source and drain to thereby applya high voltage to the memory gate as a write format relative to thenonvolatile memory cell, the electric field between the source andmemory gate of a write non-selected nonvolatile memory cell that sharesthe memory gate control lines between the write non-selected nonvolatilememory cell and a write selected memory cell does not increase inparticular if the source potential for causing the current to flowbetween the source and drain of the write selected memory cell issupplied via the corresponding source line. If the source potential isof a low source potential for write non-selection, then there is apossibility that a large electric field comparable to at erasure willact between the source and memory gate of the write non-selected memorycell that shares the memory gate control lines between the writenon-selected memory cell and the write selected memory cell. Adisturbance occurs that such a large electric field undesirably changesthe threshold voltage of a memory cell placed in a write state. Theabove relationship between the number of the memory gate control linesshared by the memory gate driver and the number of source lines sharedby the source driver is of use in preventing the fear of such adisturbance beforehand.

The memory gate driver and the source driver may be driven based on theoutput of an OR circuit which forms the OR of selected states withrespect to their corresponding plural control gate control lines. Atthis time, an input stage of the OR circuit may be used with transistorsusing extended portions of the control gate control lines as gateelectrodes thereof in order to reduce a layout area of the OR circuit.

In terms of the speeding up of a read operation, a plurality of chargeMOS transistors for respectively causing the memory gate control linesto be conducted to the first power supply voltage in response to theread operation may be provided at different positions of the memory gatecontrol lines. The time necessary to cause each memory gate control lineto transition to a desired level in terms of the read operation can beshortened.

Further, in order to control the threshold voltage of the correspondingmemory cell so as to fall within a predetermined voltage distribution, awrite verify operation may be performed after a write operation, and anerase verify operation may be carried out after an erase operation.

[4] The essential points of the present invention will be listed here interms of the device structure of each memory cell. All the essentialpoints may not be necessarily provided and are effective singly or invarious combinations. Incidentally, a prerequisite for the presentinvention is that a gate electrode to which a high voltage is appliedupon writing/erasure, and a gate electrode of a selection MOS typetransistor are configured with being separated from each other. (1) Thethickness of a gate insulating film of each selection MOS typetransistor is set thinner than that of a high-voltage MOS transistorwhich handles write/erase voltages to thereby increase Gm of theselection MOS type transistor. The thickness of the gate insulating filmof the selection MOS type transistor is set so as to be equal to that ofa gate oxide film of a MOS type transistor having charge of a logicaloperation unit (core-logic) or an I/O MOS type transistor which handlesthe input/output of a signal from and to the outside, in the case wherethe thickness of the gate insulating film is thinnest. Further, the gateelectrode of the selection MOS type transistor is driven by itscorresponding core-logic MOS type transistor operated at high speed. (2)A diffusion layer of each selection MOS type transistor constituting acell is shared with a diffusion layer of the core-logic or I/O MOS typetransistor having its gate oxide film to thereby suppress a shortchannel effect. Further, a diffusion layer of a memory holding MOS typetransistor is caused to have a junction withstand voltage higher thanthat for the diffusion layer of the selection MOS type transistor. (3) Ap type density of a channel impurity for determining the threshold valueof the selection MOS type transistor is set so that the threshold valueof the transistor becomes positive, and set thicker than that of thememory holding MOS type transistor. In the memory holing MOS typetransistor, a neutral threshold value thereof is made negative such thatthe threshold value at erasure becomes sufficiently low and a readcurrent is obtained on a large scale. The p type density of the channelimpurity is set lower than that of the selection MOS type transistor.Alternatively, in order to set the neutral threshold value of the memoryholding MOS type transistor to be negative, an n type impurity densityof its channel is made higher than an n type impurity density of achannel of the selection MOS type transistor whose threshold value ispositive.

Thus, an improvement in read speed of a semiconductor nonvolatile memorydevice can be achieved. Accordingly, the semiconductor nonvolatilememory device can be provided for high-speed program reading. If asemiconductor integrated circuit device using the technology of thepresent invention is used, then a high-performance information apparatuscan be realized at low cost. The present invention is effective for aportable device or the like free of space in which a temporary storagememory device capable of high-speed reading is built in.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing one example of a nonvolatilememory cell used in the present invention;

FIG. 2 is an explanatory view typically depicting characteristics withrespect to the nonvolatile memory cell shown in FIG. 1;

FIG. 3 is an explanatory view illustrating by way of example thresholdvoltage states where erase and write states of the nonvolatile memorycell are set to depletion and enhancement types;

FIG. 4 is an explanatory view illustrating by way of example thresholdvoltage states where the erase and write states of the nonvolatilememory cell are both set to the enhancement type;

FIG. 5 is an explanatory view showing, as comparative examples, severalconnection forms related to the nonvolatile memory cell shown in FIG. 2,prior to its optimization;

FIG. 6 is an explanatory view illustrating by way of example a devicesection, operating voltages and a hierarchical bit line structurerelated to a stack gate type flash memory cell having a floating gate;

FIG. 7 is an explanatory view illustrating by way of example a devicesection, operating voltages and a hierarchical bit line structurerelated to a split gate type flash memory cell;

FIG. 8 is an explanatory view illustrating by way of example a devicesection, operating voltages and a hierarchical bit line structurerelated to a MONOS-stack gate type memory cell ofone-transistor/one-memory cell;

FIG. 9 is an explanatory view illustrating by way of example a devicesection, operating voltages and a hierarchical bit line structurerelated to a NONOS type memory cell of two-transistor/one-memory cell;

FIG. 10 is a cross-sectional view showing a device section whereattention is given to a write operation of the nonvolatile memory cellshown in FIG. 2;

FIG. 11 is a cross-sectional view showing the manner in which a voltageapplied state analogous to a write voltage state of FIG. 10 is given toa structure of a nonvolatile memory cell made up of a series circuit ofmemory holding MONOS and selection MOS transistors;

FIG. 12 is a plan view illustrating by way of example a planarconfiguration of the nonvolatile memory cell shown in FIG. 1;

FIG. 13 is a plan view illustrating by way of example a planarconfiguration of each of the nonvolatile memory cells shown in FIGS. 6and 8;

FIG. 14 is a plan view illustrating by way of example a planarconfiguration of the nonvolatile memory cell shown in FIG. 7;

FIG. 15 is a plan view illustrating by way of example a planarconfiguration of the nonvolatile memory cell shown in FIG. 9;

FIG. 16 is a circuit diagram showing one example of a memory cell arraywhich adopts the nonvolatile memory cell shown in FIG. 1;

FIG. 17 is a circuit diagram depicting one example of a memory cellarray in which ZMOSs are constituted by CMOS transfer gates;

FIG. 18 is a circuit diagram showing one example of a memory cell arraywhich adopts sub bit line discharge transistors;

FIG. 19 is a circuit diagram illustrating by way of example the layoutof drivers with respect to the memory cell arrays each of which adoptsthe nonvolatile memory cell shown in FIG. 1;

FIG. 20 is a circuit diagram showing one example of a memory cell array;

FIG. 21 is a circuit diagram illustrating another example of the memorycell array;

FIG. 22 is a circuit diagram showing a further example of the memorycell array;

FIG. 23 is a timing chart illustrating by way of example operationtimings at the time that the direction of current at a read operation ofa nonvolatile memory cell extends from a source line to a bit line;

FIG. 24 is a block diagram of a microcomputer in which a nonvolatilememory having adopted the nonvolatile memory cells is provided on-chip;

FIG. 25 is a block diagram showing a detailed one example of a flashmemory module;

FIG. 26 is a circuit diagram illustrating by way of example a form of aforward read operation with respect to a nonvolatile memory cell;

FIG. 27 is a timing chart illustrating by way of example main signalwaveforms at the forward read operation of FIG. 26;

FIG. 28 is a circuit diagram illustrating by way of example a form of abackward read operation with respect to a nonvolatile memory cell;

FIG. 29 is a timing chart illustrating by way of example main signalwaveforms where a read operation is started as the backward readoperation of FIG. 28 after a main bit line on the input side of a senseamplifier has been precharged;

FIG. 30 is a timing chart illustrating by way of example main signalwaveforms where a read operation is started as the backward readoperation of FIG. 28 without precharging the main bit line on the inputside of the sense amplifier;

FIG. 31 is an explanatory view illustrating by way of example otherwrite voltage conditions and the like with respect to the nonvolatilememory cell;

FIG. 32 is a circuit diagram showing another example illustrative of thelayout of a memory cell array having adopted the nonvolatile memorycells, and drivers;

FIG. 33 is a circuit diagram illustrating by way of example a circuitformat in which memory gate control lines are individually driven bytheir corresponding drivers according to the selection of control gatecontrol lines as shown in FIG. 19;

FIG. 34 is a circuit diagram principally showing a drive format ofmemory gate control lines corresponding to FIG. 32;

FIG. 35 is an explanatory view showing in detail a state in whichvoltages are applied to a memory cell in an allowable disturb state;

FIG. 36 is a circuit diagram illustrating by way of example aconfiguration which needs routing of control gate control lines as adrive form of memory gate control lines;

FIG. 37 is a circuit diagram illustrating by way of example a specificconfiguration of a logic circuit;

FIG. 38 is a plan view illustrating by way of example a layoutconfiguration of a NOR gate;

FIG. 39 is an explanatory view illustrating by way of example thedifference between effects obtained according to whether source-linecoupled MOS transistors are adopted;

FIG. 40 is a cross-sectional view of a memory cell according to a firstembodiment of the present invention;

FIG. 41 is a diagram for describing the operation of the memory cellaccording to the first embodiment of the present invention and voltagesapplied thereto;

FIG. 42 is a cross-sectional view showing a state in which the memorycell according to the first embodiment of the present invention is mixedwith other MOS transistors;

FIG. 43 is a cross-sectional view of a memory cell according to a secondembodiment of the present invention;

FIG. 44 is a diagram for describing the operation of the memory cellaccording to the second embodiment of the present invention and voltagesapplied thereto;

FIG. 45 is a cross-sectional view of a modification of the memory cellaccording to the second embodiment of the present invention;

FIG. 46 is a cross-sectional view showing the difference in channeldensity in the memory cell according to the second embodiment of thepresent invention;

FIG. 47 is a cross-sectional view of a memory cell according to a thirdembodiment of the present invention;

FIG. 48 is a cross-sectional view of a memory cell according to a fourthembodiment of the present invention;

FIG. 49 is a cross-sectional view of a memory cell according to a fifthembodiment of the present invention;

FIG. 50 is a first cross-sectional view related to a process formanufacturing a semiconductor integrated circuit in which a memory cellaccording to the present invention is mixed with other MOS typetransistors;

FIG. 51 is a second cross-sectional view related to the process formanufacturing the semiconductor integrated circuit in which the memorycell according to the present invention is mixed with other MOS typetransistors;

FIG. 52 is a third cross-sectional view related to the process formanufacturing the semiconductor integrated circuit in which the memorycell according to the present invention is mixed with other MOS typetransistors;

FIG. 53 is a fourth cross-sectional view related to the process formanufacturing the semiconductor integrated circuit in which the memorycell according to the present invention is mixed with other MOS typetransistors;

FIG. 54 is a fifth cross-sectional view related to the process formanufacturing the semiconductor integrated circuit in which the memorycell according to the present invention is mixed with other MOS typetransistors;

FIG. 55 is a sixth cross-sectional view related to the process formanufacturing the semiconductor integrated circuit in which the memorycell according to the present invention is mixed with other MOS typetransistors;

FIG. 56 is a seventh cross-sectional view related to the process formanufacturing the semiconductor integrated circuit in which the memorycell according to the present invention is mixed with other MOS typetransistors;

FIG. 57 is an eighth cross-sectional view related to the process formanufacturing the semiconductor integrated circuit in which the memorycell according to the present invention is mixed with other MOS typetransistors;

FIG. 58 is a circuit diagram illustrating by way of example aconfiguration of a memory array to which the memory cells each accordingto the present invention are applied;

FIG. 59 is a cross-sectional view of a memory cell according to a sixthembodiment of the present invention;

FIG. 60 is a first cross-sectional view related to a process formanufacturing the memory cell according to the sixth embodiment of thepresent invention;

FIG. 61 is a second cross-sectional view related to the process formanufacturing the memory cell according to the sixth embodiment of thepresent invention;

FIG. 62 is a third cross-sectional view related to the process formanufacturing the memory cell according to the sixth embodiment of thepresent invention; and

FIG. 63 is a cross-sectional view of a memory cell according to aseventh embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows one example of a nonvolatile memory cell (hereinafter alsosimply called memory cell). The nonvolatile memory cell 1 includeswithin a p type well region 2 provided over a silicon substrate, a MOStype first transistor section 3 used for information storage, and a MOStype second transistor section 4 (selection MOS transistor section)which selects the first transistor section 3. The first transistorsection 3 includes an n type diffusion layer (n type impurity region) 10which serves as a source line electrode connected to a source line, acharge storage region (e.g., silicon nitride film) 11, insulating films(e.g., silicon oxide film) 12 and 13 disposed over the front and backsurfaces of the charge storage region 11, a memory gate electrode (e.g.,n type polysilicon layer) 14 for applying a high voltage upon writingand erasure, and an oxide film (e.g., silicon oxide film) 15 forprotection of the memory gate electrode. The insulating film 12 isformed 5 nm thick, the charge storage region 11 is formed 10 nm thick(silicon oxide film conversion), and the oxide film 13 is formed 3 nmthick. The second transistor section 4 has an n type diffusion layer (ntype impurity region) 16 which serves as a bit line electrode connectedto a bit line, a gate insulating film (e.g., silicon oxide film) 17, acontrol gate electrode (e.g., n type polysilicon layer) 18 and aninsulating film (e.g., silicon oxide film) 9 which insulates the controlgate electrode 18 and the memory gate electrode 14 from each other.

Assuming that the sum of the thickness of the charge storage region 11of the first transistor section 3 and the thicknesses of the insulatingfilm 12 and the insulating film 13 disposed over the front and backsurfaces thereof (called memory gate insulating films 11, 12, 13together) is represented as tm, the thickness of the gate insulatingfilm 17 of the control gate electrode 18 is represented as tc, and thethickness of the insulating film between the control gate electrode 18and the charge storage region 11 is represented as ti, the relationshipof tc<tm≦ti is realized. A gate withstand voltage of the secondtransistor section 4 is set lower than that of the first transistorsection 3 by the difference in dimension between the gate insulatingfilm 17 and the memory gate insulating films 11, 12 and 13. A planarconfiguration of the nonvolatile memory cell 1 shown in FIG. 1 isillustrated by way of example in FIG. 12.

Incidentally, the term of the drain (drain) described in part of thediffusion layer 16 means that the diffusion layer 16 functions as adrain electrode of a transistor upon a data read operation, whereas theterm of the source (source) described in part of the diffusion layer 10means that the diffusion layer 10 functions as a source electrode of thetransistor upon the data read operation. Upon erase/write operations,the functions of the drain electrode and the source electrode might becounterchanged with respect to the notation of the drain (drain) andsource (source).

FIG. 2 typically shows characteristics with respect to the nonvolatilememory cell shown in FIG. 1. A connection form of the nonvolatile memorycell 1 in a hierarchical bit line structure is illustrated by way ofexample in FIG. 2. The diffusion layer 16 is connected to a sub bit lineBL (hereinafter also simply called bit line BL), the diffusion layer 10is connected to a source line SL, the memory gate electrode 14 isconnected to a memory gate control line ML, and the control gateelectrode 18 is connected to a control gate control line CL. The sub bitline BL is connected to a main bit line (also called global bit line) GLvia an n channel type switch MOS transistor (ZMOS) 19. Although notshown in the figure in particular, a plurality of the nonvolatile memorycells 1 are connected to the sub bit line BL, and a plurality of the bitlines BL are connected to one main bit line GL via the ZMOSs 19.

A first driver (word driver) 21 which drives the control gate controlline CL, a second driver 22 which drives the memory gate control lineML, a third driver (Z driver) 23 which switch-drives the ZMOS 19, and afourth driver 24 which drives the source line SL, are typically shown inFIG. 2. The drivers 22 and 24 are constituted by high voltage MOSdrivers using MOS transistors whose gate withstand voltages are highvoltages. The drivers 21 and 23 are constituted by drivers using MOStransistors whose gate withstand voltage are relatively low.

Upon a write operation for setting a relatively high threshold voltageto the first transistor section 3 of the nonvolatile memory cell 1, forexample, a memory gate voltage Vmg and a source line voltage Vs arerespectively set to high voltages, 1.8V is supplied as a control gatevoltage Vcg, a write selection bit line is set to 0V (circuit's groundpotential) and a write non-selection bit line is set to 1.8V. In thisstate, the second transistor section 4 for the write selection bit lineis turned on to cause current to flow from the diffusion layer 10 to thediffusion layer 16. Hot electrons developed in the vicinity of thecharge storage region 11 on the control gate electrode 18 side by thecurrent may be retained in the charge storage region 11. When wiring iscarried out using a constant current ranging from about a fewmicroamperes to about several tens of microamperes as the write current,the potential of the write selection bit line is not limited to theground potential. For example, about 0.8V may be applied thereto to feeda channel current. Upon the write operation, the diffusion layer 10functions as the drain and the diffusion layer 16 functions as thesource in the n channel type memory cell. Such a write format results insource side injection of the hot electrons.

Upon an erase operation for setting a relatively low threshold voltageto the first transistor section 3, for example, a high voltage isapplied as the memory gate voltage Vmg to discharge the electronsretained in the charge storage region 11 into the memory gate electrode14. At this time, the diffusion layer 10 is set to the circuit's groundpotential. At this time, the second transistor section 4 may be broughtto an on state.

As apparent from the above write/erase operations effected on the firsttransistor section 3, the above operation can be realized withoutapplying the high voltage to the control gate control line CL and thebit line BL. This guarantees that the gate withstand voltage of thesecond transistor section 4 may be relatively low. The ZMOS 19 needs notto have a high withstand voltage either

As illustrated by way of example in FIG. 3, although not restricted inparticular, the first transistor section 3 held in an erase state inwhich its threshold voltage is rendered low, is set to a depletion type,and the first transistor section 3 held in a write state in which itsthreshold voltage is made high, is set to an enhancement type. In theerase and write states of FIG. 3, the memory gate electrode 14 at a readoperation may be set to the circuit's ground potential. Further, forexample, a power supply voltage Vdd may be added to the memory gateelectrode 14 when the read operation is speeded up. On the other hand,when both of erase and write states are set to an enhancement type asshown in FIG. 4, for example, a power supply voltage Vdd is added to thememory gate electrode 14 at a read operation. Under both the thresholdstates of FIGS. 3 and 4, the MOS type second transistor section 4 thatselects the first transistor section 3, is provided in association withthe MOS type first transistor section 3 used for information storage inthe case of the present invention. Therefore, there is no need toperform a verify (verification) operation for writing and erasure. Whenit is necessary to relax stress to the memory cell due to the write anderase operations, e.g., the number of times that writing is carried outis increased, the verify operation may be allowed.

Upon the read operation for the nonvolatile memory cell 1 of FIG. 2 inthe threshold state of FIG. 3, the source line voltage Vs and the memorygate voltage Vmg may be set to 0V, and the control gate voltage Vcg ofeach memory cell to be read and selected may be set to a select level of1.8V. When the second transistor section 4 is brought to an on state,memory information is read into the corresponding bit line BL accordingto whether current flows in accordance with the threshold voltage stateof the first transistor section 3. Since the second transistor section 4is thinner than the first transistor section 3 in gate oxide filmthickness and also lower than it in gate withstand voltage, the wholecurrent supply capacity of the nonvolatile memory cell 1 can berelatively enlarged as compared with the case in which both the memoryholding MOS transistor and selection MOS transistor are formed at a highwithstand voltage, thus making it possible to speed up a data readspeed.

Upon the read operation for the nonvolatile memory cell 1, the directionof current can be set to the direction (backward direction) opposite tothe forward direction. As illustrated by way of example in FIG. 23, asource line voltage Vs is set to 1.8V and a bit line voltage Vd is setto 0V. A control gate voltage Vcg is set to a select level (1.8V) at atime t0 of FIG. 23 with respect to each read and selected memory cell.Since the memory cell is rendered low in threshold voltage because it isin the erase state in the case of this example, the bit line voltage Vdrises. A change in this voltage is detected by an unillustrated senseamplifier.

A form of a forward read operation with respect to the nonvolatilememory cell 1 is illustrated by way of example in FIG. 26 in the form ofa circuit diagram. Iread indicates the direction of a read current. Mainsignal waveforms at the time of feeding of Iread are illustrated by wayof example in FIG. 27. A form of a backward read operation with respectto the nonvolatile memory cell 1 is illustrated by way of example inFIG. 28 in the form of a circuit diagram. Main signal waveforms at thistime are illustrated by way of example in FIGS. 29 and 30. FIG. 29 showsa case in which GL on the input side of a sense amplifier is prechargedand thereafter the read operation is started, and FIG. 30 shows a casein which the read operation is started without precharging GL on theinput side of a sense amplifier. The sense amplifiers shown in FIGS. 26and 28 may be a differential input type. In this case, a reference inputof each sense amplifier is a voltage between the high voltage side andthe low voltage side of a memory Vth on the data input side in each ofFIGS. 27, 29 and 30.

Several connected states related to the nonvolatile memory cell 2 shownin FIG. 2, prior to its optimization are shown in FIG. 5 as comparativeexamples. A comparative example 1 relative to the present inventionshows a mode in which the direction (Iprog) of a write current is madeopposite to the present invention. Since, in this case, a write highvoltage 6V must be applied to a sub bit line BL, there is a need to setZMOS as a high-voltage MOS transistor and set a Z driver as ahigh-voltage MOS driver. In the hierarchical bit line structure, theoperation of reading memory information becomes slow as compared withthe form of the present invention.

A comparative example 2 shows a configuration wherein a first transistorsection 3 is connected to a sub bit line BL and a second transistorsection 4 is connected to a source line SL to thereby cause a writecurrent to flow from the source line SL side to the sub bit line BLside. Since a write high voltage is applied to the source line SL sidein this case, there is a need to configure the second transistor section4 as a high withstand-voltage structure and configure a word driver as ahigh-voltage MOS driver. In this respect, the present example is unfitfor speeding up of the read operation.

A comparative example 3 shows a configuration wherein a first transistorsection 3 is connected to a bit line BL and a second transistor section4 is connected to a source line SL to thereby cause a write current toflow from the bit line BL side to the source line SL side. There is aneed to configure a ZMOS and Z driver as high-voltage MOS transistors ina manner similar to the comparative example 1 even in this case. In thisrespect, the present example is unfit for speeding up of a readoperation.

In order to speed up the read operation of the nonvolatile memory cell1, as apparent from FIG. 5, the first transistor section 3 is connectedto the source line SL, the second transistor section 4 is connected tothe bit line BL, and the direction of the write current is set so as toextend from the source line SL side to the bit line BL side. Thisresults in the optimum condition.

FIGS. 6 through 9 illustrate nonvolatile memory cells different from thenonvolatile memory cell shown in FIG. 2, as comparative examples. FIG. 6illustrates by way of example a device section, operating voltages, anda hierarchical bit line structure related to a stack gate type flashmemory cell having a floating gate. FIG. 7 illustrates by way of examplea device section, operating voltages, and a hierarchical bit linestructure related to a split gate type flash memory cell. FIG. 8illustrates by way of example a device section, operating voltages, anda hierarchical bit line structure related to a 1 Tr (Transistor)/1 MC(Memory Cell)-type MONOS (Metal Oxide Nitride Oxide Semiconductor) stackgate type flash memory cell. FIG. 9 illustrates by way of example adevice section, operating voltages, and a hierarchical bit linestructure related to a 2 Tr/1 MC-type MONOS type memory cell. A planarconfiguration of each of the nonvolatile memory cells shown in FIGS. 6and 8 is illustrated by way of example in FIG. 13, a planarconfiguration of the nonvolatile memory cell shown in FIG. 7 isillustrated by way of example in FIG. 14, and a planar configuration ofthe nonvolatile memory cell shown in FIG. 9 is illustrated by way ofexample in FIG. 15.

The stack gate type, the split gate type and the MONOS type respectivelyneed to apply high voltages as bit line voltages Vd or control gatevoltages Vcg upon write and erase operations. Thus, since high voltagesare applied to a MOS transistor constituting a word driver for driving acontrol gate electrode, a MOS transistor section directly below thecontrol gate electrode of a memory cell, a ZMOS connected to its drain,and a MOS transistor constituting a Z driver for driving the ZMOS, asnecessary, they are constituted using thick-film high-voltage MOStransistors. These thick-film high-voltage MOS transistors are containedin a read path and rate-control a read speed. Accordingly, the use ofthose nonvolatile memory cells makes it difficult to execute ahigh-speed read operation.

The erase operation of the stack gate type nonvolatile memory cell shownin FIG. 6 results in the operation of pulling out electrons from afloating gate FG storing the electrons to a substrate by F-N tunneling.The write operation thereof results in the operation of storingelectrons in the floating gate FG by hot electrons. The erase operationof the split gate type nonvolatile memory cell shown in FIG. 7 resultsin the operation of pulling out electrons from a floating gate FGstoring the electrons to a control gate by F-N tunneling. The writeoperation thereof results in the operation of storing electrons in thefloating gate FG by injection of hot electrons produced at the end ofthe control gate into the source side. The erase operation of the 1 Tr/1cell-type MONOS type nonvolatile memory cell shown in FIG. 8 results inthe operation of pulling out electrons from a nitride film N of a chargestorage layer ONO storing the electrons to a control gate by F-Ntunneling. The write operation thereof results in the operation ofapplying a voltage to a source terminal (interchanging the source anddrain in a transistor level) and storing hot electrons in the nitridefilm N of the charge storage region ONO. The erase operation of the 2Tr/1 cell-type MONOS type nonvolatile memory cell results in theoperation of applying a negative voltage to a memory gate to therebypull out F-N tunnel electrons from a nitride film N of a charge storagelayer ONO storing the electrons to a substrate. The write operationthereof results in the operation of storing electrons in the nitridefilm N of the charge storage layer ONO with F-N tunneling by thepotential of a channel just below the memory gate.

It is apparent that any of the stack gate type, split gate type andMONOS type mentioned as the comparative examples shown in FIGS. 6through 9 interposes the high-voltage MOS transistor in a memoryinformation read path and has a limitation of speeding up of the readoperation.

A device section at the time that attention has been given to the writeoperation of the nonvolatile memory cell shown in FIG. 2, is shown inFIG. 10. In a write voltage state shown in the figure, a channel of 6Vis formed up to the neighborhood of the control gate electrode 18 justbelow the charge storage region 11. On the other hand, a channel justbelow the control gate electrode 18 is 0V. Thus, a steep electric field(sudden electric field) is formed just below the memory gate electrode18 of the charge storage region 11 to make it possible to control acurrent that flows through a source-to-drain channel. Hot electrons areproduced by the sudden electric field and stored in the charge storageregion 11. Since the channel just below the control gate electrode 18 is0V, the insulating film 17 of the control gate electrode 18 isguaranteed to be thinned to the extent identical or substantially equalto a majority of MOS transistors such as logic circuits free of highwithstand voltages, etc. When the current is narrowed down, the channeljust below the control gate electrode 18 is about 0.8V.

The reason why the channel just below the control gate electrode 18 isnot set to 6V upon the write operation, is that no high-density impurityregion, e.g., diffusion layer is formed between the bit line electrode16 and the source line electrode 10 formed in the well region 2.Assuming that a structure of a nonvolatile memory cell (equivalent toMONOS of FIG. 9) made up of a series circuit of a memory holding MONOSand a selection MOS transistor is configured as one example asillustrated by way of example in FIG. 11, series-connected nodes of bothtransistors are configured as a diffusion region (source-drain region)30 common to both. When the common diffusion region 30 common to boththe transistors is interposed therebetween, a high voltage at writing isapplied to the MONOS to form a channel, so that the high voltage on theMONOS side is applied to the selection MOS transistor from the channelvia the diffusion region 30 common to the two transistors. In FIG. 11showing the voltage applied state analogous to the write voltage stateof FIG. 10, a voltage close to 5V is applied to the diffusion layer 30lying between the control gate electrode 31 and the memory gateelectrode 32, and hot electrons are produced at a drain end and taken orbrought in a charge storage layer 33. Since the diffusion layer 30between the control gate electrode 31 and the memory gate electrode 32results in 5V, an insulating film for the control gate electrode 31needs to reach the thickness of an insulating film for each high-voltageMOS. Hence the read operation at high speed cannot be performed. As isthe case even in FIG. 9, it is essential that the selection MOStransistor is of a high-withstand voltage type in the case of the MONOStype memory cell.

One example of a memory cell array having adopted the nonvolatile memorycells 1 is shown in FIG. 16. The nonvolatile memory cells 1 are arrangedin matrix form with 1024 rows×2048 columns and share source lines SL inunits of 16 rows×2048 columns. 64 nonvolatile memory cells correspondingto one column are connected to a sub bit line BL and are respectivelyconnected to a main bit line GL via a p channel ZMOS 19 p. When a pchannel type MOS transistor is adopted as the ZMOS 19 p, a propagationsignal level is not reduced a threshold voltage before and after theZMOS 19 p. It is thus possible to satisfactorily cope with a voltagereduction in read signal level to the corresponding bit line LB.

However, even if an attempt to set the bit line LB to a circuit's groundpotential (0V) is made when the writing or erasure of the nonvolatilememory cell 1 is performed, the potential of the bit line does not reacha level lower than the threshold voltage of the p channel type ZNOS 19p. In order to solve it, CMOS transfer gates may be adopted wherein pchannel type ZMOSs 19 p and n channel type ZMOSs 19 n are connected inparallel as illustrated by way of example in FIG. 17. The ZMOSs 19 p and19 n constituting the CMOS transfer gates are respectivelyswitch-controlled by decode signals of address decoders. Logicdesignated at numeral 40 means the final decode output stage of theaddress decoder.

Another example of a memory cell array having adopted the nonvolatilememory cells 1 is shown in FIG. 18. In the example shown in the samefigure, n channel type discharge MOS transistors 20 n switch-operatedcomplementarily to p channel type ZMOSs 19 p are provided at theircorresponding sub bit lines LB. Thus, when the sub bit line LB isselected via the ZMOS 19 p, the sub bit line LB is perfectly dischargedby the corresponding discharge MOS transistor 20 n. It is thereforepossible to prevent the level of a main bit line GL precharged beforethe start of reading from undesirably varying, stabilize a senseoperation of a read signal by a differential sense amplifier or thelike, and make a contribution to the speeding up of a read operation.The ZMOS 19 p and the discharge MOS transistor 20 n areswitch-controlled by a decode signal of an address decoder. Logicdesignated at numeral 41 means the final decode output stage of theaddress decoder.

The layout of drivers associated with the memory cell arrays each havingadopted the nonvolatile memory cells 1 is illustrated by way of examplein FIG. 19. Each of the memory cell arrays 50 and 50 has such aconfiguration as explained in FIG. 16, for example. The first driver 21and third driver 23 are disposed on one side and the second driver 22and fourth driver 24 are disposed on the other side, with the two memorycell arrays 50 and 50 being interposed therebetween. The first driver 21and the third driver 23 respectively receive an address decode signal 51so that their operations are selected. The second driver 22 and thefourth driver 24 respectively receive an output signal 52 of the firstdriver 21 so that their operations are selected. It is thus possible toseparate drivers each operated with a high voltage as an operatingsource or power supply and circuits each operated with a relatively lowvoltage as an operating power supply from side to side.

Incidentally, the write operation can be performed in control gate ormemory gate units in the configuration shown in FIG. 19. At this time,source lines are shared with memory cells corresponding to 16 rows asunits. Further, the source lines are drive-controlled using OR signalson sixteen control gate control lines 52. Prog is a write operationcontrol signal.

Within the memory arrays 50 and 50, memory gate control lines ML arerespectively formed integrally with memory gate electrodes andconstituted by laminating low resistance metal layers MGmt overpolysilicon layers MGps. The polysilicon layers MGps and the lowresistance metal layers MGmt are brought into contact at suitable spots.Control gate control lines CL are also formed integrally with controlgate electrodes and constituted by laminating low resistance metallayers CGmt on polysilicon layers CGps. The polysilicon layers CGps andthe low resistance metal layers CGmt are also brought into contact atsuitable positions. With the adoption of a shunt structure whereinpolysilicon wirings are backed in the low resistance metal layers,wiring resistance can be reduced.

Discharge MOS transistors 53 for causing the memory gate control linesML to be conducted to a circuit's ground potential Vss (0V) in responseto a read operation are provided at different positions of the memorygate control lines ML. Even if a relatively large delay componentproduced due to the parasitic capacitance and wiring resistance or thelike of each memory gate control line ML exists, the memory gate controlline ML can be rapidly discharged to 0V for the purpose of the readoperation, so rapid transition to a read operation enable state isenabled.

In the configurations of the memory cell arrays 50 described in FIGS. 16through 19, byte (8-bit) writing is enabled as a write unit. Further,since the write current is small as compared with the normal hotelectrons because of the source side injection as mentioned above,writing in control gate control line (word line) units such as 128 bytesis also enabled. Though erase units are basically used as the word lineunits, the erase units may be units that use the source lines SLillustrated by way of example in FIG. 16 in common. Alternatively, unitsof a plurality of source lines SL may be batch-collected.

Incidentally, when defect relief in a word line direction is taken intoconsideration, the unit of the defect relief results in the unit thatuses at least the source line SL in common. In order to carry out thedefect relief, although not shown in the drawing in particular, thereare provided a relief memory array replaced with a defective portion, arelief address program circuit which stores an address to be relievedthrough a nonvolatile memory or the like, an address comparator whichcompares the address stored in the relief address program circuit withan access address, and a relief selection circuit. When the result ofcomparison by the address comparator indicates that they coincide witheach other, the relief selection circuit prohibits accessing based onthe access address and operates the relief memory array by use of arelief address related to the coincidence as an alternative to it.

FIGS. 20 through 22 show other sectional structures of the nonvolatilememory cell 1 according to the present invention. As illustrated by wayof example in FIG. 20, a charge storage region 11 and a memory gateelectrode 14 may be disposed above a control gate electrode 18. As shownin FIG. 21, a charge storage region 11 and a memory gate electrode 14are disposed adjacent to a control gate electrode 18, and the memorygate electrode 14 may be formed as a sidewall gate. Alternatively, asshown in FIG. 22, a control gate electrode 18 may be formed as asidewall gate.

Although not shown in the figure in particular, the charge storageregion 11 is not limited to adoption of a charge trapping insulatingfilm covered with an insulating film, like the above-described siliconnitride film (silicon nitride film). As an alternative to it, aconductive floating gate electrode (e.g., polysilicon electrode) coveredwith an insulating film, or a conductive particle layer or the likecovered with an insulating film may be adopted. The conductive particlelayer can be constituted by, for example, nanodots in which polysiliconis formed in dot form.

A whole configuration of a semiconductor device, e.g., a microcomputerin which a nonvolatile memory having adopted the nonvolatile memorycells described above is provided on-chip, is shown in FIG. 24. Althoughnot restricted in particular, the microcomputer 60 is formed on onesemiconductor substrate (semiconductor chip) like monocrystal silicon bya CMOS integrated circuit manufacturing technology. The microcomputer 60includes a CPU (Central Processing Unit) 61, a RAM 62 which serves as avolatile memory, a flash memory module 63 which serves as a nonvolatilememory, a flash memory controller 64, a bus state controller 65, aninput/output circuit (I/O) 66 such as an input/output port circuit orthe like, and other peripheral circuit 67. Those circuit modules areconnected to an internal bus 68. The internal bus 68 includes signallines respectively used for addresses, data and control signals. The CPU61 includes an instruction controller and an execution unit, and decodesa fetched instruction and performs arithmetic processing according tothe result of decoding. The flash memory module 63 stores an operationprogram for the CPU 61 and data therein. The RAM 62 serves as a workarea or a data temporary storage area for the CPU 61. The operation ofthe flash memory module 63 is controlled based on control data set tothe flash controller 64 by the CPU 61. The bus state controller 65controls the number of access cycles, waist state insertion, a buswidth, etc. with respect to access and external bus access via theinternal bus 68.

A circuit indicated by an area 69 surrounded by a chain double-dashedline in FIG. 24 means a circuit portion constituted by MOS transistorsrelatively thin in gate oxide film. Circuits other than the area 69result in circuit portions constituted by high-voltage MOS transistorsrelatively thick in gate oxide film. As the circuit portion, ismentioned, for example, an area formed with high-voltage drivers 22 and24, etc. in the flash memory module 63.

A detailed one example of the flash memory module is shown in FIG. 25. Amemory array 70 has such configurations as described in FIGS. 16 and 19and the like. A driver circuit 71 is a circuit block provided with thedrivers 23 and 21, etc. A driver to be output-operated in accordancewith a code signal is selected by the corresponding address suppliedfrom an X address decoder (XDCR) 73. A driver circuit 72 includes thedrivers 22 and 24, etc. A driver to be output-operated is selected inaccordance with a control gate control line CL state or the like. Asense amplifier circuit and write controller 78 is connected to globalbit lines GL. The sense amplifier circuit 78 amplifies and latches readdata read out into the corresponding global bit line GL. The writecontroller 78 latches write control information to be supplied to thecorresponding global bit line upon a write operation. The senseamplifier circuit and write controller 78 is connected to a datainput/output buffer (DTB) 80 via a Y selection circuit (YG) 79 and ishence capable of interfacing with a data bus 68D included in theinternal bus 68. Upon a read operation, the Y selection circuit 78selects read data latched in the sense amplifier circuit 78 inaccordance with an address decode signal outputted from a Y addressdecoder (YDCR) 74. The selected read data can be outputted to theoutside via the data input/output buffer 80. Upon the write operation,the Y selection circuit 78 controls to which global bit line write datasupplied from the data input/output buffer 80 is caused to correspondand whether the write data is latched in the write controller 78.

An address signal is supplied from an address bus 68A to an addressbuffer 75 from which it is supplied to the X address decoder 73 and theY address decoder 74. A voltage generator (VS) 77 generates operatingpower supplies necessary for reading, erasure and writing, based onexternal power supplies Vdd and Vss. Considering write operatingvoltages shown in FIG. 2, for example, they result in Vdd=1.8V,VCCE=12V, VCCP=8V and VCCD=6V.

A controller (CONT) 76 performs a control sequence of a read operation,an erase operation and a write operation of the flash memory module 63and switching control on the operating power supplies therefor inaccordance with the control information set to the flash memorycontroller 64. The switching control on the operating power suppliescorresponds to control for switching the operating power supplies of thedrivers 21 through 24 according to the operating modes of FIG. 2 inaccordance with the read operation, erase operation and write operation.

Other write voltage conditions and the like with respect to thenonvolatile memory cell are illustrated by way of example in FIG. 31. Abasic difference between FIG. 31 and FIG. 2 resides in that the powersupply voltage Vdd is changed from 1.8V to 1.5V, and the memory gatevoltage Vmg at reading is changed from 0V to Vdd=1.5V. Further, thesource voltages Vs and memory gate voltage Vmg at writing (Program) anderasure (Erase) are also changed. A hierarchical structure using ZMOSs19 is not adopted for each bit line BL in FIG. 31. The adoption of sucha bit line hierarchical structure as shown in FIG. 2 is allowed. Even inFIG. 31, an effect similar to FIG. 2 can be obtained.

Another example related to the layout of a memory cell array havingadopted the nonvolatile memory cells 1, and drivers is shown in FIG. 32.In the example of FIG. 32 in a manner similar to FIG. 19, the controlgate drivers 21 respectively receive address decode signals so thattheir operations are selected, whereas memory gate drivers 22A andsource drivers 24 are respectively provided so that their operations areselected based on the outputs of the control gate drivers 21. Thecontrol gate drivers 21 are disposed on one side and the memory gatedrivers 22A and source drivers 24 are disposed on the other side, with amemory array 50 being interposed therebetween. Thus, drivers eachoperated with a high voltage as an operating power supply and circuitseach operated with a relatively low voltage as an operating power supplyare separated from one another. A configuration described till here isidentical to that shown in FIG. 19, and the points of difference betweenFIG. 32 and FIG. 19 will be explained below. Incidentally, since thememory gate driver 22A increases in drive load with respect to thememory gate driver 22, it may have drive capacity larger than that ofthe memory gate driver 22 where the memory gate driver 22A falls shortof it.

The first point of difference is that memory gate control lines ML arecommonly connected in plural units. That is, the memory gate controllines ML paired with the control gate control lines CL share the use ofthe memory gate driver 22A in sixteen units, for example. The sourcelines SL also share the use of the source driver 24 in sixteen units,for example. At this time, the number Nm1 of the memory gate controllines ML shared by the memory gate driver 22A satisfies a condition(Nm1≦Ns1) that it is set less than or equal to the number Ns1 of thesource lines SL shared by the source driver 24.

The above conditions result from the following reasons. When the currentis caused to flow between the source and drain to thereby apply a highvoltage to the memory gate electrode 14 as a write format relative tothe nonvolatile memory cell, the electric field between the source andmemory gate of a write non-selected nonvolatile memory cell that sharesthe memory gate control lines ML between the write non-selectednonvolatile memory cell and a write selected memory cell does notincrease in particular if the source potential Vs for causing thecurrent to flow between the source and drain of the write selectedmemory cell is applied to its corresponding source line SL. In theexamples shown in FIGS. 31 and 32, the difference in potential betweenthe source and memory gate is about 6V. If the source potential is of alow source potential for write non-selection, then a large potentialdifference like 12V is formed between the source and memory gate of thewrite non-selected memory cell that shares the memory gate control linesbetween the write non-selected memory cell and the write selected memorycell. There is a possibility that this will bring about the action of alarge electric field comparable to at erasure. A disturbance occurs thatsuch a large electric field undesirably changes the threshold voltage ofa memory cell placed in a write state. The above relationship betweenthe number of the memory gate control lines ML shared by the memory gatedriver 22A and the number of source lines SL shared by the source driver24 is of use in preventing the fear of such a disturbance beforehand.

The reasons thereof will be further described in detail. FIG. 33illustrates by way of example a circuit format in which memory gatecontrol lines ML are individually driven by their corresponding drivers22 in a one-to-one correspondence relation between the memory gatecontrol line and the control gate control line as shown in FIG. 19. Adisturbed one is only a memory cell designated at A lying in the samerow in which the memory gate control line ML is shared with a writeselected memory cell. Since 6V is applied to a source line SL in amanner similar to the write selected memory cell even in the case of thememory cell indicated by A, only a potential difference of about 6Voccurs between the source and memory gate thereof. Therefore, such alarge electric field as produced upon erasure does not occurs, so thisresults in an allowable disturbance. If other memory cells are differentfrom the write selected memory cell in memory gate control line ML evenif the source line is commonly connected to each write selected memorycell in FIG. 33, only an electric filed opposite in direction to that atthe erasure occurs between the source and memory gate, so that adisturbance at writing does not occur. A voltage applied state of thememory cell in the state of the above-described allowable disturbance isrepresented as shown in FIG. 35 if described in detail. In the case ofFIG. 34 corresponding to FIG. 32, the potential difference of about 6Voccurs between the source and memory gate in each of memory cellsdesignated at B and C in a manner similar to the memory cell designatedat A. However, this is the same as the memory cell designated at A inFIG. 33 and hence only the allowable disturbance occurs. This is becausewhen the number of memory gate control lines ML selected for writing islarger than the number of source lines for write selection in FIG. 34, apotential difference of 12V is formed, in the direction of the sameelectric field as at erasure, between the source and memory gate of eachmemory cell connected to the write non-selected source line and thewrite selected memory gate control line from the relationship in which0V is applied to the write non-selected source line and 12V is appliedto the write selected memory gate line, whereby the memory cell isplaced in a state comparable to that at erasure.

Under the above condition Nm1≦Ns1, the memory gate driver 22A is sharedbetween the memory gate control lines ML and the source driver 24 isshared between the source lines SL, whereby a chip occupied area formedby the drivers 22A and 24 can be significantly reduced as illustrated byway of example in FIG. 34. In FIG. 33, the memory gate drivers 22 aredisposed 1024 with respect to 1024 control gate control lines CL,whereas in FIG. 34, they may be provided 64 equal to one-sixteenth the1024 memory gate drivers.

The second point of difference resides in a logic structure forgenerating drive control signals used for the memory gate driver 22A andthe source driver 24. That is, as shown in FIG. 32, the memory gatedriver 22A and the source driver 24 are configured so as to be drivenbased on the output of an OR circuit 90 for forming ORing of selectedstates corresponding to their corresponding sixteen control gate controllines CL0 through CL15. At this time, in order to reduce the routing ofthe control gate control lines CL0 through CL15, the OR circuit 90 isdisposed close to a memory array 50, and a OR result signal CLout0thereof is supplied to the drivers 22A and 24. Thus, such wire routingas illustrated by way of example in FIG. 36 becomes unnecessary.Further, there is no need to extend control gate control lines CL0through CL15 in the vicinity of a driver 22A and input them to an ORcircuit 100.

A specific configuration of the OR circuit 90 is shown in FIG. 37. TheOR circuit 90 comprises a NOR (NOR) gate. In the drawing, a NOR gate 91is interposed in its subsequent stage. The NOR gate 91 is not used withrespect to instructions for a read operation by a signal Read, and avoltage Vdd is applied to all memory gates. In other words, the NOR gate91 is used for erase and write operations other than the read operation.

In order to reduce a layout area of the NOR gate 90 in particular,extended portions of control gate control lines CL0 through CL15 areconstituted using transistors Q0 through Q15 used as gate electrodes. Alayout configuration of the NOR gate 90 is illustrated by way of examplein FIG. 38.

The third point of difference resides in an improvement made from theviewpoint of speeding up of the read operation. That is, as illustratedby way of example in FIG. 32, a plurality of charge MOS transistors 92for causing the memory gate control lines ML0, . . . to be conductive tothe power supply voltage Vdd in response to the read operation arerespectively provided at different positions of the memory gate controllines ML. The time required to cause the corresponding memory gatecontrol line ML to transition to a desirable level from the readoperation viewpoint can be shortened. The charge MOS transistor 92 isbrought to an on state in response to a state for instructing rewritedisable by a signal SWE.

Further, as illustrated by way of example in FIG. 32, there are providedMOS transistors 95 for causing the source lines SL0, . . . that sharethe use of the source driver 24 in sixteen units, to be conducted to acircuit's ground potential in response to the read operation.Furthermore, there are provided coupling MOS transistors 94 forselectively bringing groups of the source lines SL set in sixteen unitsinto conduction. As illustrated by way of example in FIG. 39, thecoupling MOS transistors 94 and the MOS transistors 95 are turned onupon the read operation to thereby make it possible to apparently make aresistance reduction in the source line SL. The MOS transistors 94 and95 are respectively brought to an on state in response to a state forgiving instructions for non-writing by a signal P in FIG. 32.

FIG. 40 is a cross-sectional view showing a first embodiment of thepresent invention, which is illustrative of a memory cell using afloating gate. The memory cell comprises a p type well region PWELprovided over a silicon substrate, an n type diffusion layer MS whichserves as a source region, an n type diffusion layer MD which serves asa drain region, a floating gate FLG, a tunnel oxide film FTO, aninterlayer insulating film INTP, a memory gate electrode MG (material: ntype polysilicon) for applying a high voltage upon writing/erasure, anoxide film CAP for protection of the memory gate electrode MG, a gateoxide film STOX for a selection MOS type transistor, a selection gateelectrode SG made of n type polysilicon, and an insulating film GAPOXfor insulating the selection gate electrode SG and the memory gateelectrode MG. The gate oxide film STOX is characterized in that thethickness of the gate oxide film STOX is fabricated thinner than that ofthe insulating film GAPOX and that of a high-voltage MOS type transistorfor writing/erasure. The gate oxide film STOX and the insulating filmGAPOX are respectively formed by other layers. The interlayer insulatingfilm INTP may be a generally-used laminated structure of oxidefilm/nitride film/oxide film of silicon in a floating gate type.

FIG. 41 shows the operation of the cell disclosed in FIG. 40 and how toapply voltages thereto. Here, the injection of charges into the floatinggate FLG is defined as write (Program). A write system indicates hotelectron writing using source side injection. A voltage Vs applied tothe source region MS is 5 volts. A voltage Vmg applied to the memorygate electrode MG is 10 volts. A voltage Vsg applied to the gateelectrode SG of the selection MOS type transistor is made substantiallyidentical to the threshold value of the MOS type transistor. Ageneration region of the hot electrons is a channel portion below aGAPOX region by which the two gate electrodes are isolated from eachother, in a manner similar to FIG. 40.

In the case of ejection of charges from the floating gate FLG, whichfunctions as an erase operation, such an electric field as to dischargeor eject stored charges (electrons) toward the p type well region PWELis generated. When the potential difference is set as 20V, for example,the voltage Vmg applied to the memory gate electrode MG is set to −20volts, and the voltage Vwell applied to the p type well region PWEL isset to 0. Alternatively, the voltage Vmg applied to the memory gateelectrode MG is set to −10 volts, the voltage Vwell applied to the ptype well region PWEL is set to 10V, and the voltage Vsg applied to thegate electrode SG is set to 10 volts. The voltage Vsg applied to thegate electrode SG is a voltage necessary to eliminate the difference inpotential between the gate electrode SG and the p type well region PWELand avoid damage of the gate oxide film STOX.

If the voltage to the source/drain at reading is applied in thedirection opposite to that at writing where the operating voltage ofmixed core logic is 1.8 volts, then the voltage Vs applied to the sourceregion MS is set to 0 volt, the voltage Vs applied to the drain regionMD is set to 1.8 volts, and the voltage Vsg applied to the gateelectrode SG is set to 1.8 volts. If, at this time, the threshold valueof a memory in an erase state is set enough lower than 0, then thevoltage Vmg applied to the memory gate electrode MG can be read at 0volt. In the case of forward reading, the voltage Vd applied to thedrain region MD may be set to 1.8 volts and the voltage Vs applied tothe drain region MS may be set to 0. As one having a high potential forbeing mixed in addition to the core logic, is mentioned, MOS typetransistors for I/O each of which handles a signal inputted from outsideand outputted to the outside. They cope with voltages higher than thosehandled by the core logic, e.g., 3.3 volts, 2.5 volts, etc. Thethickness of a gate insulating film of each of these MOS typetransistors for I/O is thinner than the insulating film GAPOX. In thecase of 3.3 volts, the thickness thereof is approximately 8 nanometers,whereas in the case of 2.5 volts, the thickness thereof is about 6nanometers. Since the thickness thereof is thinner than that of theinsulating film GAPOX that needs a high withstand voltage, these may beadopted as the thickness of the gate oxide film STOX. As read voltagesto be applied, may be used 1.8 volts mentioned previously, or 3.3 voltsor 2.5 volts for I/O.

Both the memory cell illustrative of the first embodiment shown in FIG.40 and other MOS type transistors mixed therein are shown in FIG. 42 insectional structure. Those newly added to the sectional structure ofFIG. 42 as notation are device isolation regions SGI, a p type wellLPWEL for a core logic n MOS type transistor (Core Logic MOS), a gateoxide film LVGOX thereof, a gate electrode LVG thereof, a source/drainregion LVSD thereof, a p type well HPWEL for a write/erase high-voltageMOS type transistor, a gate oxide film HVGOX thereof, a gate electrodeHVG thereof, a source/drain region HVSD thereof, a wiring interlayerinsulating film INSM1, a wiring M1 a for supplying a low output voltageof the core logic MOS type transistor to the selection gate electrode SGwithin a first wiring layer, and a wiring M1 b for supplying a highoutput voltage of the write/erase MOS type transistor to the memory gateelectrode MG. Although an upper wiring further exists in practice, it isomitted in the present figure.

If the gate oxide films STOX, LVGOX and HVGOX, and a tunnel oxide filmFTO are all defined as a silicon oxide film and the physical thicknessesof those are respectively defined as tS, tL, tH and tF, then therelationship of tL≦tS<tF<tH is established in the nonvolatile memorydevice according to the present invention. Although the description of asectional view of the MOS type transistor for I/O is omitted, therelationship of tL<tIO<tF is established if the thickness of its gateinsulating film is defined as tIO. Even if the same thickness as thethickness tIO is adopted as the thickness tS, the relationship oftL≦tS<tF<tH is maintained and is capable of falling into the scope ofthe present invention. If theses films are not constituted of thesilicon oxide film alone, e.g., a nitride film is used in part thereof,then the thickness relationship that characterizes the present inventioncan be generalized down to the electrical thickness from the physicalthickness. Because the structure and thickness of the gate insulatingfilm are set corresponding to the respective applied voltages and matchwith the feature of the present invention in which a thicknessconstitution based thereon is applied even to a cell structure.

Speaking of other feature of the present invention in terms of therelationship of connection between the MOS type transistor and thememory cell, the selection gate electrode SG and the source/drain regionLVSD of the MOS type transistor for the core logic are directlyconnected to each other by the wiring layer M1 a, and the memory gateelectrode MG and the source/drain region HVSD of the write/erase MOStype transistor are directly connected to each other by the wiring layerM1 b.

FIG. 43 is a cross-sectional view showing a second embodiment where thepresent invention is applied to a MONOS type memory cell that performsdiscrete charge storage. FIG. 43 is different from FIG. 40 in that alaminated structure is configured wherein the charge storage regionresults in a nitride film SIN of silicon, an oxide film BOTOX is formedjust below the nitride film SIN, and an oxide film TOPOX is formed justabove the oxide film BOTOX. The thickness of the nitride film SIN is setto less than or equal to 50 nanometers. Assuming that the thickness ofthe oxide film TOPOX is defined as tT and the thickness of the oxidefilm BOTOX is defined as tB, both tT and tB are set so as to reach arelationship of tB>tT where stored charges are pull out via the oxideTOPOX, whereas both tT and tB are set so as to reach a relationship oftB<tT where the stored charges are pulled out via the oxide film BOTOX.The relationship of the film thickness described in FIG. 42, i.e.,tL≦tS<tF<tH is similar in either case. While the silicon nitride filmhas been illustrated by way of example as the charge storage layer inthe present embodiment, the present invention can be applied even in thecase of other insulating trap film, e.g., alumina or the like.

FIG. 44 shows the operation of the cell disclosed in FIG. 43 and how toapply voltages thereto. FIG. 44 is basically identical to FIG. 41 butthe voltage Vmg applied to the memory gate electrode MG is set to 12volts where electrical charges are ejected toward the memory gateelectrode MG and erased. This is a voltage applying method where thethickness of the oxide film BOTOX is thicker than that of the oxide filmTOPOX. The voltage Vmg applied to the memory gate electrode MG is set to−12 volts where electrical charges are discharged into the p type wellregion PWEL and erased. This is a voltage applying method where thethickness of the oxide film BOTOX is thinner than that of the oxide filmTOPOX. Incidentally, the absolute value 12 volts of the erase voltage isshown as one example. The present invention is not limited by thisnumerical value.

FIG. 45 illustrates by way of example a source/drain structure employedin the memory cell shown in FIG. 43. As a premise, the operatingvoltages correspond to the write, erase and reverse read shown in FIG.44. In this case, the junction withstand voltage of the drain may beidentical to a 1.8V-operated CMOS (MOS type transistor for core logic).Accordingly, a drain region can adopt the same structure as one for corelogic. That is, the drain region is configured as an LDD structurecomprising a low density region MDM and a high density region MD and canbe shared with a source/drain region of the MOS type transistor for thecore logic. Thus, since a short channel effect of a selection MOS typetransistor can be suppressed, its gate length can be shortened. This isfit to obtain a large read current under a low voltage. On the otherhand, the drain region to which a high voltage is applied upon writing,cannot make use of the same structure as the source/drain region of thecore logic transistor and hence results in a double drain structurecomprising a high-density region MS and a diffusion layer MSM for animprovement in withstand voltage. A source region thereof can also beshared with a source/drain region of a high-voltage MOS type transistorfor write/erase voltage control, and may be configured as a structurededicated for the memory cell as needed.

FIG. 46 shows the difference in channel density between the selectionMOS type transistor and memory MOS type transistor in the memory cellshown in FIG. 43. In order to secure a large read current under a lowvoltage, the lower the threshold value of the MOS type transistor thebetter. However, when the threshold value of the selection MOS typetransistor becomes extremely low, the selection MOS type transistor isnot turned off perfectly even where its gate voltage is 0. This leads tothe fact that a leak current inhibits a normal read operation.Accordingly, the threshold value of the selection MOS type transistormay preferably be low within a positive range. On the other hand, thereis a need to sufficiently lower the threshold value of the memory MOStype transistor in order to set the read current high. In order toenable long-time storage of charges, the voltage of the memory gateelectrode MG at reading may preferably be set to 0 volt. Thus, if apremise is made that no leak occurs in the selection MOS typetransistor, then there is a need to make negative the threshold value ofthe memory MOS type transistor placed in an erase state.

The conventional floating gate type is capable of obtaining a sufficientlow threshold value by increasing an erase voltage or applying the erasevoltage for a long period of time. However, the memory cell using thetrap-acting film as in the present embodiment has the characteristicthat the threshold value is not reduced to a constant value or less.Therefore, it should be necessary to adjust the channel density and setlow the original threshold value in order to lower the threshold valueof the memory MOS type transistor. If a channel impurity density is setsuch that a neutral threshold value becomes negative, then thepost-erasure threshold value can be also made negative. If such asetting is done, then a large read current value can be obtained whenthe selection MOS type transistor is turned on to read a signal.Accordingly, the difference must be inevitably provided between theimpurity density of a channel region SE of the selection MOS typetransistor and the impurity density of a channel region ME of the memoryMOS type transistor. If a comparison is made between a p type impuritydensity Nse of the channel region SE and a p type impurity density Nmeof the channel region ME in the case of the memory cell formed on the ptype well region PWEL as shown in FIG. 46, then the respective impuritydensities are set such that the relationship of Nse>Nme is established.Alternatively, the p type impurity densities are set identical but an ntype impurity density of the ME region is set higher than an n typeimpurity density of the SE region. The difference in density is one ofthe essential points of the present invention, which aims to obtain alarge read current under a low voltage.

Incidentally, since the amount of change in threshold value issuppressed low in an uncontrolled state where the threshold value of thememory MOS type transistor is set negative upon the use of theconventional floating gate type, there is a need to suppress low theneutral threshold value of the memory MOS type transistor. When thevoltage of the memory gate electrode MG is set to 0 upon reading, theneutral threshold value thereof may also preferably be set negative.Accordingly, the above-described essential point is establishedregardless of a charge storing method for the memory MOS typetransistor.

FIG. 47 is a cross-sectional view showing a third embodiment where thepresent invention is applied to a memory cell using fine particles as acharge storage portion. Fine particles DOTS are provided on anunderbedding oxide film BOTOX. As the material of each fine particleDOTS, may be mentioned polysilicon. Another material may be used. Thediameter of the particle may preferably be 10 nanometers or less. Aninterlayer insulating film INTOX is deposited so as to cover up the fineparticles DOTS, and a memory gate electrode MG is mounted justthereabove. The charge storage portion may be considered to be equal toa charge storage film having a trap property in that it becomesdiscrete. Accordingly, the contents of the present invention describedup to now can be applied in any combination thereof or all combinationthereof.

The relationship of tL≦tS<tH and the cell structure corresponding to thegist of the present invention have been described above. A specificmanufacturing method will be explained below with reference to asectional view showing a fourth embodiment of FIG. 48. Disclosed hereinis a sectional structure wherein a sidewall spacer GAPSW formed byanisotropic dry etching is used as the insulating film GAPOX forinsulating the selection gate electrode SG and memory gate electrode MGemployed in the second embodiment. The sidewall spacer GAPSW is formedby a technique similar to a method for forming a sidewall spacer usedwhere an LDD (Lightly Doped Drain) structure of a diffusion layeremployed in a normal MOS type transistor is formed. However, an oxidefilm formed immediately after dry etching is reduced in withstandvoltage due to etch damage. Since the applied voltage is low and thereare not provided electrodes adjacent via the sidewall spacer in the caseof the normal CMOS, the reduction in withstand voltage becomes almosttrivial. However, when the sidewall spacer is used as the insulatingfilm GAPOX for isolating the selection gate electrode SG and the memorygate electrode MG from each other, it is necessary to ensure a withstandvoltage of about 15 volts. It is thus essential to carry out anannealing process in an oxygen atmosphere for the purpose of animprovement in withstand voltage prior to deposition of polysilicon usedas the selection gate electrode SG after a silicon oxide film depositedover the whole surface is etchbacked by anisotropic dry etching to formthe sidewall spacer GAPSW. This is a process indispensable to therealization of the memory cell structure according to the presentembodiment.

FIG. 49 is a cross-sectional view of a fifth embodiment wherein the gateelectrodes of the selection MOS type transistors each shown in FIG. 48are configured as a structure self-aligned with a memory section. Thisstructure is one formed by depositing a gate electrode material (e.g.polysilicon) over the whole surface and effecting an anisotropic dryetching process on it. Gate electrodes of selection MOS type transistorsformed by such a technique correspond to SGR1 and SGR2 in the drawing.They are different in shape from the gate electrode SG of FIG. 48 havingthe same function but the difference therebeween resides in that pointalone. That is, the relationship of tL≦tS<tH and the like correspondingto the gist of the present invention remains unchanged.

Cross-sectional views related to a manufacturing process at the timethat the above memory cell according to the present invention and otherMOS type transistors are mixed, are disclosed in FIGS. 50 through 56.

FIG. 50 will first be explained. Device isolation oxide film regions SGIare formed over a p type silicon substrate PSUB to form a p type wellPWL for an n type MOS type transistor (nMOS) for core logic, an n typewell NWL for a p type MOS type transistor (pMOS) for the core logic, a ptype well HPWL for a high-voltage control n type MOS type transistor(nHVMOS) for writing/erasure, an n type well HNWL for a high-voltagecontrol p type MOS type transistor (pHVMOS), and an n type well MWL fora memory cell region. Next, an impurity for controlling the thresholdvalues of the respective MOS type transistors is introduced into regionswhich serve as channel surfaces. Consequently, an nMOS impurity layerNE, a PMOS impurity layer PE, an nHVMOS impurity layer HNE, a pHVMOSimpurity layer HPE, and an impurity layer ME for a memory MOS typetransistor are formed.

FIG. 51 will next be described. The surface of the silicon substrate iscleanly processed. Afterwards, a lower oxide film BOTOX (5 nanometers)of the memory MOS type transistor is formed by thermal oxidation, and asilicon nitride film SIN (15 nanometers) is deposited just thereabove bya chemical vapor deposition method. Thereafter, the surface of thesilicon nitride film SIN is subjected to thermal oxidation processing tothereby form an upper oxide film TOPOX (2 nanometers). Subsequently, ann type polysilicon layer NMG (100 nanometers) which serves as a memorygate electrode later, and a silicon oxide film CAP (100 nanometers) forprotection of the memory gate electrode MG are sequentially deposited.

FIG. 52 will next be explained. The laminated films BOTOX, SIN, TOPOX,NMG and CAP corresponding to the five layers formed over the siliconsubstrate in FIG. 51 are processed into shapes of gate electrodes MG1and MG2 of the memory MOS type transistor using a photolithographytechnology and a dry etching technology. They are represented as linearshapes long in a depth direction as viewed in the figure. They exist bythe same number as the number of word lines. However, only two lines aretypically shown on the drawing. Upon processing, dry etching is stoppedat the stage of exposure of the surface of the lower oxide film BOTOX,and the remaining lower oxide film BOTOX is removed by hydrofluoricacid. This is a method for preventing unwanted etching damage to thesubstrate surface. Owing to this hydrofluoric acid processing, thesubstrate surface is exposed. Subsequently, a thermal oxide film BOX (5nanometers) is formed and a silicon oxide film HVGOX (15 nanometers) isdeposited thereon. Afterwards, the oxide films corresponding to the twolayers are provided for the gate oxide films of the high-voltage controlMOS type transistors. Since only the mere deposited films degradereliability, a laminated structure is adopted.

FIG. 53 will next be explained. The resultant structure is processed bythe photolithography technology to form a photoresist film RES1 whichcovers a region for forming the MOS type transistors for the core logicand a region for forming the high-voltage control MOS type transistors.Thereafter, the oxide film that exists in the channel region of eachselection MOS type transistor, is removed by an anisotropic dry etchingtechnology effected on the silicon oxide film HVGOX to thereby exposethe substrate surface. According to this process, sidewall spacers GAPSWobtained by processing the silicon oxide film HVGOX is also formedsimultaneously on each selection MOS type transistor side of the memoryMOS type transistor. Subsequently, an impurity layer SE forthreshold-value control is formed in the channel region of eachselection MOS type transistor while the photoresist film RES1 is beingleft behind. The impurity densities of the impurity layer SE and theimpurity layer ME satisfy the relationship disclosed in FIG. 46.

FIG. 54 will next be explained. A photoresist film RES2 is processed bythe photolithography technology to open only the region for forming thecore-logic MOS type transistors. Afterwards, the oxide films of thelaminated structure comprising the thermal oxide film BOX and thesilicon oxide film HVGOX are perfectly removed by the hydrofluoric acidprocessing.

FIG. 55 will next be described. After the photoresist film RES2described in the previous drawing has been removed and the cleaningprocess has been completed, a thermal oxide film (4 nanometers) isformed over the exposed silicon substrate surface (core-logic MOS typetransistor section and selection MOS type transistor section). Thethermal oxide film results in a gate oxide film LVGOX for the core-logicMOS type transistors and a gate oxide film STOX for each selection MOStype transistor. Although the respective gate oxide films for thecore-logic MOS type transistors and the selection MOS type transistorsare indicated as separate symbols LVGOX and STOX in the present drawingfor convenience, the thicknesses of the two become identical if thepresent manufacturing method is taken. Subsequently, a non-dopedpolysilicon film (150 nanometers) is deposited over the whole surface.Afterwards, impurities are introduced into the polysilicon film in sucha manner that an n type is formed on each of the regions for forming thenMOS and nHVMOS, and a p type is formed on each of the regions forforming the pMOS and pHVMOS. The densities of the impurities arerespectively set to 1×10²⁰/cm³ or more. Subsequently, a silicon oxidefilm (20 nanometers) is deposited over the whole surface. Afterwards,the laminated film of the polysilicon film and the silicon oxide film isprocessed using the photolithography technology and the dry etchingtechnology to thereby form a gate electrode LVGn for the nMOS, a gateelectrode LVGp for the pMOS, a gate electrode HVGn for the nHVMOS, and agate electrode HVGp for the pHVMOS. At this time, only the end of thegate electrode on the source side of each selection MOS type transistoris processed in the memory region. A gate length in a 0.18μ-generationresults in, for example, 0.15 micron in the core-logic MOS typetransistors and 1.0 micron in the high-voltage control MOS typetransistors HVMOS. This is however a necessary consequence due to thefact that the voltages to be handled are different from one another.Subsequently, an n type source/drain LLDDn having a shallow junction forthe nMOS, a p type source/drain LLDDp having a shallow junction for thepMOS, an n type source/drain HLDDp having a high withstand-voltagejunction for the nHVMOS, and a p type source/drain HLDDp having a highwithstand-voltage junction for the pHVMOS are appropriately formed byusing the photolithography technology and an ion-implantation technologyusing impurity ions. These sources/drains should be designed on theassumption that junction withstand voltages sufficient for the voltagesto be used are ensured. The core-logic MOS type transistors becomehigher than the high-voltage control MOS type transistors HVMOS in thedensity of each source/drain impurity introduced here. Although an ntype diffusion layer MDM is formed at the drain of each selection MOStype transistor, the densities of the impurities for the n typediffusion layer MDM and the n type source/drain LLDDn can be madeidentical to each other according to the manufacturing method disclosedherein.

FIG. 56 will next be explained. A drain region of the memory MOS typetransistor is formed in the present drawing. A photoresist film RES3having an opening with respect to the region used as the drain of thememory MOS type transistor and whose opening end is provided on thememory gate electrodes MG1 and MG2, is formed by a photolithographyprocess. Afterwards, the laminated film of the polysilicon film and thesilicon oxide film is processed by anisotropic dry etching to therebyform gate electrodes SG1 and SG2 of the two selection MOS typetransistors. Subsequently, ion-implantation of an n type impurity iseffected without removing the photoresist film RES3 to thereby form asource region MSM of the memory MOS type transistor.

FIG. 57 will next be described. A silicon oxide film (100 nanometers) isdeposited over the whole surface and subsequently the anisotropic dryetching is effected on the whole surface. Owing to this processing,spacers SWSPLDD are formed on their corresponding sidewalls of all thegate electrodes. High-density n type diffusion layers NSD and MS, andhigh-density p type diffusion layers PSD are respectively formed insources/drains of all the n type transistors and sources/drains of ptype transistors by ion implantation and heat treatment. Subsequently,the oxide films are removed from all the sources/drains NSD, MS and PSDand gate electrodes LVGn, LVGp, HVGn, HVGp, SG1 and SG2 to therebyexpose silicon. A metal cobalt (10 nanometers) is deposited over thewhole surface and subjected to heat treatment at 700° C. to thereby forma self-aligned cobalt silicide. Unreacted unnecessary cobalt is removedby cleaning, followed by execution of processing at 750° C. again,whereby a low-resistance cobalt silicide layer COSI is formed.Thereafter, an insulating oxide film INSM1 is deposited over the wholesurface. A subsequent wiring process is allowed to use the conventionalart.

FIG. 58 shows one embodiment of a memory array configured using a memorycell technology of the present invention. A basic configuration thereofis of a NOR type and takes a hierarchical bit line structure. In thepresent embodiment, two global bit lines are typically shown forsimplification. A global bit line BLP is connected to a sense amplifierSAP. The global bit line BLP has branches to local bit lines. ZAPindicates a selection MOS type transistor for selecting a local bit lineLBAP. A plurality of memory cells MPA1 through MPA4 are connected to thelocal bit line LBAP. Although four memory cells are typically shown inthe figure, the number of memory cells to be connected might be sixteen,thirty-two or sixty-four. Selection MOS type transistors of the memorycells are connected to the local bit line LBAP. The selection MOS typetransistor ZAP and the memory cells MPA1 through MPA4 are collectivelyreferred to as a block BLCPA. In a block BLCQA arranged symmetricallywith the block BLCPA, memory cells MQA1 through MQA4 are connected to alocal bit line LBAQ, and ZAQ indicates a MOS type transistor forselecting them. A global bit line corresponding to the block BLCQA isdesignated at BLQ and connected to a sense amplifier SAQ. The selectionMOS type transistors ZAP and ZAQ are MOS type transistors each havingthe same gate oxide-film thickness as each of the core-logic MOS typetransistors. A driver for transmitting a signal to gate electrodes ofthe selection MOS type transistors ZAP and ZAQ is designated at ZSLA.The driver ZSLA is also constituted of a core-logic MOS type transistor.The gate electrodes of the cell selection MOS type transistors areconnected to their corresponding word lines which extend across theblocks adjacent to each other in the horizontal direction. For example,the gate electrode of the cell selection MOS type transistor of thememory cell MPA1 that belongs to the block BLCP is connected to a wordline WAP1, whereas the gate electrode of the cell selection MOS typetransistor of the memory cell MPA2 that belongs to the block BLCQ isconnected. One for selecting the word line WAP1 is a driver WSLA1. Thisalso makes use of the core-logic MOS type transistor. Drivers WSLA2through WSLA4 are associated with word lines WAP2 through word linesWAP4 in a one-to-one relationship. The drivers WSLA1 through WSLA4 andthe driver ZSLA are collectively referred to as a driver group DECA.Memory gates also cross in the horizontal direction. MWAP1 is a wiringused in common with the memory gates of the memory cell MPA1 and thememory cell MQA1. In order to apply a high voltage upon writing/erasure,a driver MGSLA1 for supplying the voltage to the wiring MWAP1 is made upof a high-voltage MOS type transistor. Drivers MGSLA2 through MGSLA4 areassociated with wirings MWAP2 through MWAP4 in a one-to-onerelationship. There is a need to supply 5 volts to a wiring COMSL sharedbetween the block BLCPA and the block BLCQA upon writing. This is doneby a driver PRVS constituted of a high-voltage MOS type transistor. Thedrivers MSGLA1 through MSGLA4 and the driver PRVSA each comprising thehigh-voltage MOS type transistor are collectively referred to as adriver group HVDRVA. As shown in the figure, other blocks BLPB and BLQBare further connected to the global bit lines BLP and BLQ respectively.Driver groups DECB and HVDRVB corresponding to them exist. Similarly,blocks BLPC and BLQC, and driver groups DECC and HVDRVC exist. Uponreading, the individual drivers contained in the driver groups DECAthrough DECC select the word lines in accordance with addressesrespectively. Since, however, these have performance equivalent to thecore logic, the selected word line can be driven at high speed.Accordingly, the reading of information can be performed at high speed.This is a method of constituting the memory array corresponding to thememory cell structure of the present invention.

FIG. 59 shows a structure for reducing capturing of electrons at writinginto regions other than a trap film in a memory cell of the presentinvention. While the memory cell of the present invention is basicallyidentical to the memory cell described up to now, the present memorycell is characterized by the shape of an insulating film isolating aselection gate electrode SG and a memory gate electrode MG from eachother and its forming method. As shown in the drawing, the shape of theinsulating film at a sidewall portion of MG, which isolates SG and MGfrom each other, is made thick at the sidewall portion of MG, and thinat a sidewall portion of the trap film SIN. The injection of electronsby source side/injection occurs in the vicinity of an MG end close toSG. However, it is not possible to avoid that some of the electrons arestored in the insulating film for separating SG and MG from each other.Since the region for its storage does not corresponds to the originalelectron storage portion, a necessary electric field cannot be appliedupon erasure, so it is hard to discharge or eject the stored electrons.Consequently, there is a possibility that desired write and eraseoperations will be inhibited. Accordingly, this region corresponds to anallowable range of withstand voltages of SG and MG and may preferably beset as narrow as possible. Thickening only the thickness of theinsulating film in the region in which the side surfaces of SG and MGare opposite to each other makes it possible to ensure withstandvoltages for SG and MG without impairing the original write and eraseoperation.

A method of manufacturing the memory cell will be explained using FIGS.60 through 62. In FIG. 60, MG is processed by anisotropic dry etchingand thereafter an oxide film ISSGOX of abut 10 nanometers is adhered orbonded onto the whole surface by a method called ISSG oxidation. Thisoxidation method has been described in IEEE Electron Device Letters(IEEE ELECTRON DEVICE LETTERS), Volume 21, No. 9, September 2000, pp430–432. This is a technology capable of forming a thin oxide film highin withstand voltage and quality. The present technology brings aboutone characteristic even in that oxide films equal in thickness can beformed not only over a silicon surface but also over the surface of anitride film. An oxide film excellent in withstand voltage can beadhered even onto an exposed sidewall of a storage trap film.

FIG. 61 is a process following FIG. 60. Although thermal oxidation isadded after ISSGOX has been adhered, SIN sidewalls are almostnon-oxidized, and the sidewall of MG corresponding to polysilicon isoxidized thick. According to this process, the insulating film betweenSG and the storage trap film can be made thin, and the insulating filmbetween SG and MG can be made thick.

FIG. 62 is a process following FIG. 61. The surface of the siliconsubstrate is also thermal-oxidized immediately after the formation ofthe shape of FIG. 61. When the oxide film is now anisotropically etched,only the oxide film formed over the substrate surface is removed, sothat a thick oxide film GAPOX-TH of the MG sidewall can be left as anecessary insulating film shape. The surface of the silicon substrate isretreated by the removed thermal oxide film. Thereafter, a thin gateoxide film STOXR for a selection MOS type transistor may be thermallyformed after a cleaning process. Subsequently, SG, a source (MSM andMS), and a drain (MDM and MD) may be formed sequentially. They aresimilar to other executed items of the present invention. Incidentally,the structure described with reference to FIGS. 59 through 62 can beused even when any of the floating gate, trap film and conductive fineparticles is used.

FIG. 63 shows a handling method taken where a deposited oxide film isused for the gate insulating film of the selection MOS type transistor.A large amount of defects normally exist in the deposited oxide film andlead to unwanted charge storage and leak currents. A drawback arises inthat when the deposited oxide film is used as the gate insulating film,reliability becomes significantly low. An article of Kamigaki et al.published in “Journal of Applied Physics in 1996” NO. 80, pp 3430describes that a defect (E′ center) in an oxide film can be reduced byheat treatment in an oxygen atmosphere, and an interface state (Pbcenter) can be reduced by high-temperature heat treatment in a hydrogenatmosphere. If this method is used where the gate insulating film ofeach selection MOS type transistor constituting the memory cell of thepresent invention is formed by the deposited oxide film, the selectionMOS type transistor can be used as a high-reliable MOS type transistor.GAPOX may be formed using the etchback system described in FIG. 48.Thereafter, a deposited oxide film STOXCV is adhered onto the wholesurface. The deposited oxide film STOXCV is provided for insulating theselection gate electrode SG and the memory electrode MG andsimultaneously exists even just below SG. Further, the deposited oxidefilm STOXCV functions even as the gate insulating film of the selectionMOS type transistor. A procedure for performing heat treatment in anoxygen atmosphere immediately after the adhering of the STOXCV andsubsequently adhering and forming SG is executed. In the presentinvention, the heat treatment in the oxygen atmosphere, which iseffected on the STOXCV, is defined as pyrogenic oxidation at 800° C. to850° C. for 10 through 20 minutes. Thereafter, diffusion layers such asa source MS, a drain MD, etc. are formed. The high-temperature heattreatment in the hydrogen atmosphere may be performed at 700° C. to 750°C. With application of the heat treatment in the hydrogen atmosphere,electronic conductivity in a silicon nitride film can be significantlyreduced. Thus, the system used in the present invention, for locallyinjecting the hot electrons into the trap film such as the nitride filmand storing them therein results in a process important to preventdiffusion of electrons in the horizontal direction by a self-inducedelectric field. The heat treatment in the hydrogen atmosphere canachieve the most satisfactory effect by application of the heattreatment immediately before a wiring process in which other heattreatments at 700° C. have been all finished. While the STOXCV has beendescribed as the deposited oxide film corresponding to one layer, it maytake a laminated structure wherein a deposited oxide film is adheredafter the formation of a thermal oxide film or ISSG oxide film.

While the invention made above by the present inventors has beendescribed specifically based on the illustrated embodiments, the presentinvention is not limited to them. It is needless to say that variouschanges can be made thereto within the scope not departing from the gistthereof.

For example, the correspondence between the threshold voltage states andthe write/erase states with respect to the nonvolatile memory cell is arelative concept, and the definition opposite to the above may becarried out.

It is needless to say that the state of the low threshold voltage of thenonvolatile memory cell is not limited to the depletion type and may beset to the enhancement type.

Further, the write, erase and read operating voltages may suitably bechanged without being limited to ones described in FIG. 2. Upon theerase operation, no limitation is imposed on the form that the electronsin the charge storage region 11 are ejected toward the memory gate 14.The direction of the electric field at erasure is reversed and theelectrons in the charge storage region 11 may be ejected toward the wellregion 2.

The bit lines may not adopt the constitution or structure in which theyare hierarchized with respect to the global bit lines. The bit lines maybe connected to the sense amplifier circuit or write circuit. Only anyone of the sense amplifier circuit and the write circuit may be set tothe above-described hierarchized structure. Further, the power supplyvoltage, write and erase high voltages, etc. may suitably be changed tofurther other voltages.

The film thickness in the ONO structure of the nonvolatile memory cellmay take a combination of ones close to 3 nm (nano-meters), 26.5 nm and0 nm rather than near the channel region or a combination of 5 nm, 10 nmand 3 nm.

INDUSTRIAL APPLICABILITY

The semiconductor device according to the present invention is notlimited to a microcomputer in which a volatile memory is providedon-chip. The semiconductor device can be widely applied to a nonvolatilememory LSI such as a unitary flash memory, a further system on-chippedsystem LSI relatively large in logic scale, etc. In addition, thesemiconductor device according to the present invention is applicableeven to a memory card based on IDE (Integrated Device Electronics) usinga nonvolatile memory, ATA (AT Attachment), etc.

1. A semiconductor device comprising: a plurality of nonvolatile memorycells each including, a MOS type first transistor section used forinformation storage, and a MOS type second transistor section whichselects the first transistor section, wherein the first transistorsection and the second transistor section are formed with an insulatingregion between gate electrodes thereof and adjacent to each other,wherein a channel region connected to channel regions of the firsttransistor section and the second transistor section is provided belowthe insulating region, wherein the second transistor section has a bitline electrode connected to a bit line, and a control gate electrodeconnected to a control gate control line, wherein the first transistorsection has a source line electrode connected to a source line, a memorygate electrode connected to a memory gate control line, and a chargestorage region disposed directly below the memory gate electrode,wherein a gate withstand voltage of the second transistor section islower than a gate withstand voltage of the first transistor section, andwherein a gate withstand voltage of a drive MOS transistor connected tothe control gate control line is lower than a gate withstand voltage ofthe first transistor section.
 2. A semiconductor device according toclaim 1, wherein the bit line electrode and the source line electrodeare formed in a well region in which a high-density impurity region isnot formed therebetween.
 3. A semiconductor device according to claim 2,wherein the high-density impurity region is a diffused region of animpurity.
 4. A semiconductor device according to claim 1, wherein thecharge storage region is a conductive floating gate electrode coveredwith an insulating film.
 5. A semiconductor device according to claim 1,wherein the charge storage region is a charge trap insulating filmcovered with an insulating film.
 6. A semiconductor device according toclaim 1, wherein the charge storage region is a conductive fine particlelayer covered with an insulating film.
 7. A semiconductor deviceaccording to claim 1, further including: switch MOS transistors eachcapable of connecting the bit line to a global bit line, wherein thethickness of a gate oxide film of the switch MOS transistor is thinnerthan the thickness of a gate oxide film of the first transistor section.8. A semiconductor device according to claim 7, further including: afirst driver which has the drive MOS transistor and drives the controlgate control line; a second driver which drives the memory gate controlline; a third driver which drives the switch MOS transistor to an onstate; and a fourth driver which drives the source line, wherein thefirst driver and the third driver use a first voltage as an operatingpower supply, and the second driver and the fourth driver use a voltagehigher than the first voltage as an operating power supply.
 9. Asemiconductor device according to claim 8, further including: a controlcircuit which sets an operating power supply of the first driver to afirst voltage, sets an operating power supply of the fourth driver to asecond voltage higher than the first voltage, and sets an operatingpower supply of the second driver to a third voltage greater than orequal to the second voltage when a threshold voltage of the firsttransistor section is rendered high, thereby enabling injection of hotelectrons into the charge storage region from the bit line electrodeside.
 10. A semiconductor device according to claim 9, wherein thecontrol circuit sets the operating power supply of the second driver toa fourth voltage greater than or equal to the third voltage when thethreshold voltage of the first transistor section is rendered low,thereby ejecting electrons from the charge storage region to thecorresponding memory gate electrode.
 11. A semiconductor deviceaccording to claim 10, wherein the first transistor section whosethreshold voltage is made low, is set to a depletion type, and the firsttransistor section whose threshold voltage is made high, is set to anenhancement type.
 12. A semiconductor device according to claim 9,wherein when information stored in the nonvolatile memory cell is read,the control circuit sets the operating power supply of the first driverto a first voltage and sets the memory gate electrode and the sourceline electrode to a circuit's ground potential.
 13. A semiconductordevice according to claim 9, wherein when information stored in thenonvolatile memory cell is read, the control circuit sets the operatingpower supply of the first driver to a first voltage and sets the memorygate electrode and the bit line electrode to a circuit's groundpotential.
 14. A semiconductor device according to claim 8, furtherincluding a logic operation unit which performs a logical operation withthe first voltage as an operating power supply.
 15. A semiconductordevice according to claim 11, wherein each of the first driver and thethird driver receives an address decode signal so that an operationthereof is selected, and each of the second driver and the fourth driverreceives the output of the first driver so that an operation thereof isselected.
 16. A semiconductor device according to claim 15, wherein thefirst driver and the third driver are disposed on one side and thesecond driver and the fourth driver are disposed on the other side, withan array of the nonvolatile memory cells being interposed therebetween.17. A semiconductor device according to claim 16, wherein in the arrayof the nonvolatile memory cells, memory gate control lines are formedintegrally with memory gate electrodes, and low resistance metal layersare laminated over polysilicon layers respectively.
 18. A semiconductordevice according to claim 16, wherein discharge MOS transistors forcausing the memory gate control lines to be conducted to a circuit'sground potential in response to a read operation are provided atdifferent positions of the memory gate control lines.
 19. Asemiconductor device according to claim 7, wherein each of the switchMOS transistors is a p channel type MOS transistor.
 20. A semiconductordevice according to claim 19, wherein n channel type discharge MOStransistors switch-operated complementarily to the switch MOStransistors are provided at their corresponding bit lines.
 21. Asemiconductor device according to claim 7, wherein the switch MOStransistors are mutually parallel-connected n channel type MOStransistors and p channel type MOS transistors which constitute CMOStransfer gates.